Add walberla as submodule
adding walberla as a submodule will no longer require to use -DWALBERLA_DIR with cmake and also make sure that we are using a specific commit of walberla which works if the current walberla master breaks
adding walberla as a submodule will no longer require to use -DWALBERLA_DIR with cmake and also make sure that we are using a specific commit of walberla which works if the current walberla master breaks