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
Remove Assert in CellAccessor::face()

The Assert checked that DoFs are distributed before allowing the use of
::face(), which is overly restrictive.

Also see https://groups.google.com/d/topic/dealii/G3Xp-Uv72_M/discussion
Merge pull request #194 from tjhei/remove_face_it_assert

Remove Assert in CellAccessor::face()
Minor documentation updates.
Also remove output files.
Give it a once over.

No functional changes, just updates to documentation and style.
Merge pull request #196 from bangerth/step-52-updates

Step 52 updates
add failing test for ghosted vectors
Allow v=0.0 for ghosted PETSc and Trilinos vectors
Allow v=0.0 for ghosted PETSc and Trilinos vectors
Merge pull request #198 from tjhei/gla_broken_ghost_vec

PETSc and Trilinos vectors with ghost entries can now be reset to zero using = 0.0
Minor doc update.

We had forgotten to update the documentation for the new manifold_id field.
Also a few other touches of TLC.
Merge pull request #199 from bangerth/doc-update-3

Minor doc update for the new manifold_id field.
Update documentation.

The text describing the Solver base class must easily be 15 years old.
I can see 3 Germans writing it, accent, language problems, and all.
More updates.
Merge pull request #200 from bangerth/doc-updates-4

Update documentation.
Update a comment.
Merge pull request #201 from bangerth/doc-update-5

Update an error message.
Improved AVX-512 vectorization code path

The original version of the AVX-512 vectorization (DEAL_II_VECTORIZATION_LEVEL = 3) was not properly tested. When I did some tests with gcc, it turned out that I had to fix a few issues. Now it compiles fine and the generated assembler code looks good, but there is no hardware out to test it yet.
I also improved some code comments.