Merges the functionality of dune-fem-hpdg into dune-fem.

At the moment the new spaces are kept in namespace hpDG to avoid confusion. Both, PAdaptiveSpace and spaces from hpDG use the same adaptation interface now. There is some work left in the function folder. The newly introduced grid functions need to be investigated and discussed. Also, the BasisFunctionSet can be merged but this requires some effort.