Clear out the private member variables in the solvers

Many of the private member variables in in the ADERDGSolver,FiniteVolumesSolver, LimitingADERDGSolver can be computed. This includes cardinalities. Only store the order of approximation, number of variables and number of parameters. Compute all other cardinalities; provide getter functions.

• It would be better if the optimised solver would dismiss the "getBnd.." functions and would overwrite the existing, now virtual, "getUnknowns..." and "getData.." functions.