Check in DoFHandler::renumber_dofs() that DoFs are distributed

- This adds several Asserts to check that DoFs are distributed
before calling renumber_dofs() for active or level DoFs.
- Note that we now also require that you call distribute_dofs()
before distribute_mg_dofs().
- Updated documentation.
Merge pull request #191 from tjhei/renumber_dofs_asserts

Check in DoFHandler::renumber_dofs() that DoFs are distributed