Extract "constraint solvers" from volume variables

The computation of the secondary variables from the primary variables is currently often coded inside the volume variables' update function. For the purpose of potential code reusage, readability, and testability it is IMO better to move this functionality to constraint solver classes. Even if the computation is quite simple. This is sometimes done in the form of the computeFluidState function. However, there is no general interface for all models.

If the constraint solver is in a separate class it is much easier to write unit tests. Volume variables have a lot of dependencies but the constraint solver can easily be tested and mock object are simple to construct for values obtained from e.g. the spatial params or some physical law.