Rename DoFTools::map_global_to_patch_dof_indices() to get_dofs_on_patch().

Follow Timo's suggestion and revert the mapping from map_global_to_patch_dof_indices().
Consequently, also rename the function to get_dofs_on_patch(). Adjust the test.
Merge pull request #39 from bangerth/add-patch-generators-2

Rename DoFTools::map_global_to_patch_dof_indices() to get_dofs_on_patch().

Follow Timo's suggestion and revert the mapping from map_global_to_patch_dof_indices(). Consequently, also rename the function to get_dofs_on_patch(). Adjust the test.