Add more functions that deal with the infrastructure for solving local problems on patches around cells.

The functions DoFTools::count_dofs_on_patch() and DoFTools::map_global_dofs_to_patch_indices() help set up linear systems on patches. This is joint work with Arezou Ghesmati.
Then I'd need to move the function into the .h file which we've traditionally been pretty reluctant to do. I'd rather not do that.

However, the argument type is chosen to match the return type of the function in GridTools people are likely to use to generate the patches. So this should cover most cases.