Hardcoded "3"s in gitter_dune_pll_impl.cc

According to dune/alugrid/3d/grid.hh, ALU3dGrid is the BaseGrid for any ALUGrid, also for the dimension 2 ones.

I think that then the hardcoded "3"s in dune/alugrid/impl/duneinterface/gitter_dune_pll_impl.cc have to be reconsidered.