Feature/test compositionalflash

This is a test for the sequential compositional flash. It uses the same testing structure as the other unit tests for flashs. As such, it just prints an error but never fails. I propose fixing this for all flashes in #760 (closed).

I have two questions:

• Given a temperature and one phase pressure and either a saturation (two phase test) or mass fractions (one phase test), the test generates a reference fluid state. This is done with the implicit flashs currently (MiscibleMultiPhaseComposition for two phases, ComputeFromReferencePhase for one). Should it be done using a sequential routine? Additionally?
• The compositional flash is given the information it requires to calculate the fluid state itself and the test checks if this fluid state is identical to the reference fluid state (this is all the test does...). However, the compositional flash and the implicit flash are slightly different when it comes to non-existing phases: the sequential flash stores mass fractions for the non-existing phase as if two phases were present while the implicit ComputeFromReferencePhase flash stores mass fractions that are not physical but used to determine the phase switch. So I only compare values for existing phases now. Is this the way to go?

Resolves #739 (closed)