Avoid defining and using variable template

This removes the variable template isGridFunction<N> and replaces all uses of isGridFunction<N> and staticDegree<N> by IsGridFunction<N>::value and StaticDegree<N>::value,respectively. While this patch reverts some code modernization in dune-pdelab, it allows to compile with gcc-4.9 shipped with debian stable again which does not support variable templates. Notice, that the use of variable templates does hardly provide any benefit, because without it the code is almost the same.

Together with pdelab/dune-typetree!13 this allows to compile dune-pdelab with gcc-4.9 again. Furthermore all tests despite testpk2dinterpolation successfully pass. The latter timed out without result.