- Modified the names of the Trajectory methods to match the latest names in Component.h.
- Reintroduced the precision argument in SimTK::Xml::Elment::setValueAs<T>.

Added a place holder for the Catch2 unit test for class StatesDocument.

testStatesDocument.cpp is now being incorporated in the build, and the test case runs when "build" executed on the target "RUN_TESTS_PARALLEL".

- Moved all utility methods into a struct with local linkage.
- Streamlined code by adding `getEltValue()`. Use of this method reduced some code repetition.

- Minor changes to line formatting.

- Minor corrections/additions to comments

- Added code borrowed from testComponentInterface.cpp to generate a dummy model with a variety of states.

The code compiles and runs with all test cases passing, but this is just a starting point. De/serialization using class StatesDocument is not yet being tested.

- Removed code borrowed from testComponentInterface.cpp. That code was not the way to go. A legitimate model is needed.

I'm starting over.

- Made small improvements and corrections in the documentation.

- Now building a simple model and running a simulation.

- Added code to serialize, deserialize, and then re-serialize the state trajectory recorded during a simulation.

The code is functioning as expected!

- Changed member variable `m_states` from type std::vector<SimTK::State> to SimTK::Array_<SimTK::State>
- Added `exportToStatesDocument()`

- Added a `get` method that provides access to the underlying SimTK::Array_<SimTK::State> member variable.

- Added a method that tests equality of 2 state trajectories.

The method does not yet test equality for discrete variables or modeling options, but Qs, Us, and Zs are passing.

- Added a static method that computes maximum rounding error based on the specified precision and value that is rounded.

- Cleaned up the computation of max_eps.

- Added equality comparisons for discrete variables and modeling options.

- Added ExtendedPointToPointSpring.

Since the trajectory was changed from std::vector<State> to SimTK::Array_<State>, bounds checking behavior has changed.

SimTK::Array_<T> only checks bounds in DEBUG. In other words, an IndexOutOfRange exception will not be thrown in a release build. To account for this, I inserted an #ifdef to skip bounds checking unless in DEBUG.

Added the following types:
- bool
- int
- double
- Vec2
- Vec3
- Vec4
- Vec5
- Vec6

All tests are passing, and the .ostates file looks right.

So that discrete variables can be updated during a simulation, class ExtendedPointToPointSpring is now built on top of class ExtendedTwoPointLinearSpring. ExtendedTwoPointLinearSpring allocates auto-update discrete variables in its own `realizeTopology()` and those discrete variables are updated in its own `realizePosition()`.

The discrete variables can be added in extendRealizeTopology().
And, they can be changed in computeForce().

- Changed the allocation for discrete variables to the DefaultSystemSubsystem.
- Primarily using the subsystem index instead of the pointer to the subsystem.
- Discrete variables are being changed during simulation in ExtendedPointToPointSpring::computeForce().

All checks are now passing!

The note is serialized and deserialized.

All checks are passing.

1. Now looping over over output precision from precision = 2 to precision = 22.
2. Added the ability to change the name of a discrete state for the purpose of testing exceptions when a discrete state is not found.

- Cleaned up the code a little bit.
- Added code to deserialize with a model that has one discrete variable with a changed name. This was done to check exception handling.

Cleaned up the code.

The following checks are now performed:
- Serialization / deserialization for precision 1 to 21.
- Check that exception is thrown when model names don't match.
- Check that exception is thrown when a discrete state is not found.

- code cleanup
- added the ability to omit a discrete state

When a discrete state is omitted, an exception should be thrown but it is not.  Need to look into this.

Previous checks are still running properly.

const SimTK::Array_<SimTK::State>& StatesTrajectoryReporter::getStateArray();

- Removed StatesDocument::parseDoc().  No reason to have it.
- Simplified the name of StatesTrajectory::getUnderlyingStateArray() to getStateArray().
- Refined documentation comments and checked them for accuracy.

- Deleted unused code.
- Updated copyright dates.

- Improved documentation comments.

An error was being issued for Ubuntu and Mac builds because "OPENSIM_STATES_DOCUMENT_H_" was not commented out on line 609. I added the comment token.

In a number of instances, I changed the type from `int` to `size_t`.

Windows can handle a std::string, but Mac and Ubuntu builds are failing.

- Addressed some type warnings.
- Removed the method `ComputeMaxRoundingError()`.

I temporarily added a default constructor to see if that would resolve some errors that are occurring in the Continuous Integration.

It looks like different return types of size() methods are assumed across operating systems. I inserted a (size_t) cast to address compiler errors on Ubuntu

For reasons of performance and compatibility with Simbody, OpenSim::StatesDocument uses SimTK::Array_<>, instead of std::vector<>, as the container for states trajectories.

However, some user-facing classes, like OpenSim::StatesTrajectory, have opted to use std::vector<> as the container. I migrated OpenSim::StatesTrajectory to use SimTK::Array_<>. Unfortunately, this caused errors when bindings for Python and perhaps Java and Matlab are generated.

To maintain compatibility with existing code and simplify binding generation, this commit reverts to using std::vector as the container for state trajectories.

In addition, several new interface methods were introduced to the StatesDocument API to allow either SimTK::Array_<> or std::vector<> to be used as the container. Internally, however, all state trajectory operations are mediated by SimTK::Array_<>. This flexibility is gained at almost no computational cost by using a shallow copy constructor to convert std::vector<State> objects to SimTK::Array_<State> objects.

Now settings for `note` and `precision` are updated in the body of the constructor instead of in the initializer list.

Updated the documentation to reflect the fact that std::vector<SimTK::State> is also an acceptable form of the state trajectory for interfacing with OpenSim.

Migrated StatesTrajectoryReporter::getStateArray() to getVectorOfStateObjects()

Migrated StatesTrajectoryReporter::getStateArray() to getVectorOfStateObjects().

I added comments for developers should a demand additional supported variable types emerge.

Corrected a compile error.

…-org#3902)