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.
It could be worthwhile to generalize std::vector to a template argument (e.g. if one wants to use std::set, or a custom container class).