Add unit tests and automated testing

[BenjaminRodenberg]

We should add some easy-to-run unit tests. We will probably have to use some kind of mock testing (similar to https://github.com/precice/python-bindings), since we cannot simply run MATLAB via travis due to the license.

[BenjaminRodenberg]

See also precice/precice#409

[ghost]

A testing procedure with docker was added in #16. Currently tests the bindings by running the solverdummy and both tutorials (explicit and implicit coupling).

The testing starts from a docker image with Ubuntu 18.04 and MATLAB. The procedure for generating such image can be found in the test/0-matlab directory of the PR.

[BenjaminRodenberg]

#16 implements what we are referring to as a systemtest (see https://github.com/precice/systemtests). However, what we also need (since the systemtest only covers a few of the API functions) are comprehensive unit tests for the API. One possible implementation of this is using a mocked up preCICE SolverInterface.cpp like in the python-bindings (see https://github.com/precice/python-bindings/tree/develop/test).