Fix style issue.
Joint work with Arezou Ghesmati: Add GridTools::get_patch_around_cell().
Provide attribution.
Add some documentation.
Merge pull request #8 from bangerth/add-patch-generators

Add GridTools::get_patch_around_cell().

Implement a function GridTools::get_patch_around_cell() extracts the
set of cells that surround a single cell. This is joint work with
Arezou Ghesmati.