…stem

- UnitSystem holds the list of default units for each (compatible?) quantity type and a method (GetDefaultUnitInfo) that provides the corresponding UnitInfo given a QuantityType
- All methods in Quantity that use a UnitSystem now use the new GetDefaultUnitInfo
- Added a method (in UnitSystem) for creating derived unit systems(WithDefaultUnit), providing a new QuantityType->UnitInfo association and optionally a new BaseUnits definition (as currently used by Equals and visible in public ctor- otherwise obsolete?)

- updated the comments and invalid arguments checks
- added a few tests for those cases

a few question marks remain (marked with TODOs)

- renamed some tests, added comments here and there
- WithDefaultUnit : added a check for the compatibility between QuantityType and UnitInfo passed in (also added a test for this)

- had missed this one

- added an association (1:0..1) between UnitSystem and Unit (kept the name 'BaseUnit' instead of the previously considered 'DefaultUnit')
- added an association (1:0..n) between Unit and UnitSystem- this would be used to generate list of units for a quantity in a particular unit system (not an exhaustive list, needs a review)

- refactored the UnitSystem- moving the BaseUnits to BaseUnitSystem (SI derived from BaseUnitSystem), extended the unit mappings with the UnitSystemInfo class (containing both default and common/derived units)
- updated the JsonTypes and created the  UnitSystemInfoGenerator (TODO WRC)
- updated existing UnitSystemTests that tested for equality/BaseUnits (almost all in fact) to use the BaseUnitSystem instead (as the IEquatble/BaseUnits interface was pushed down)
- added several more unit-system mappings to the UnitDefinitions (Acceleration, AmountOfSubstance, Area, Energy & Length)
- commented (temporary) part of the Ctor_WithValueAndSIUnitSystem_ReturnsQuantityWithSIUnitOrThrowsArgum test (testing for support SI support)

- UnitSystem/BaseUnitSystem Equality contract based on the base (default) unit for each quantity
- BaseUnits for BaseUnitSystem created from the provided unit associations
- added unit system associations for all SI base types (Length, Mass, Duration etc.)
- tests for the UnitSystem construction/derivation

- updated the default units (as per https://en.wikipedia.org/wiki/Astronomical_system_of_units)
- fixed the docstring

in order to avoid breaking the namespace (UnitSystem.SI) all static instances were added to the UnitSystem.Definition (partial class)

- fixed typo in unit system for Length
- Ctor_UnitSystem_ThrowsArgumentExceptionIfNotSupported
- Ctor_WithNullUnitSystem_ThrowsArgumentNullException
- As_UnitSystem_ThrowsArgumentExceptionIfNotSupported
- As_WithNullUnitSystem_ThrowsArgumentNullException
- To_UnitSystem_ThrowsArgumentExceptionIfNotSupported
- ToUnit_WithNullUnitSystem_ThrowsNullException

- merged all upstream modifications
- fixed nullability issues in UnitSystem
- the Lazy<..> BaseUnits constructor was made protected
- marked all tests that rely solely on BaseUnits as Obsolete (with error=false)
- removed some obsolete tests from MassConcentration (tesing for default unit system units)

…mentExceptionIfNotSupported tests

where is the run button!?

This reverts commit e027e7c.

…UnitSystemWithOldDerivedUnits_IncludingTheNewBaseUnit