SubGrid::cell_global_to_local is now filled.

- Fixed incorrectly named fields in SubGrid::face_mapping and
SubGrid::node_mapping.
- Added some doxygen-documenation.
Aded RuntimeMPI::inputDomainSubsetOf for cells.
Renamed SubGrid::global_cell and global_face

- global_cell -> cell_local_to_global
- global_face -> face_local_to_global