Distinguish between a vector of size numEq and the primary variables class

see commit 266b7840. If !530 (merged) gets merged we need to distinguish between two concept for sake of generality:

• a vector of size number equations that we can use to store fluxes, storage, source and residuals for each equation and:
• a primary variables vector that contains the values of all primary variables at a d.o.f. location.

The latter might also have a state in the case of privar switch models that indicates the type of primary variables saved in this vector. The vectors have different usages but will default to the same type Dune::FieldVector<Scalar, numEq> and can be speziali0zed independent from each other if needed.