Make the dune-functions adapter Experimental::GridFunctionSpace work in distributed situations

The class Experimental::GridFunctionSpace allows to use function space bases from the dune-functions module in dune-pdelab. (dune-functions function space bases are a rough equivalent of dune-pdelab FiniteElementMaps). However, this previously only worked if only a single process was involved. This merge request improves Experimental::GridFunctionSpace enough to also allow for simple distributed computing.