Instantiate ProblemStat and ProblemInstat for YaspGridBasis explicitly
explicit template instantiation can reduce compile time for derived projects significantly. In my test for the ellipt.2d example, I got 18sec with explicit instantiation and 40sec without. In need to be checked whether this procedure can be extended to more grids and dimension, maybe by automatic source generation by cmake, i.e. generate explicit template instantiations in separate translation units for YaspGrid, AlbertaGrid, UGGrid, and ALUGrid for different dimension and LagrangeBasis of different degrees. These are 4x2x(3^n) instantiations. This, probably, should be made an non-default cmake option.