Clarify documentation and verify what I say by adding tests.
Merge pull request #156 from bangerth/clarify-docs

Clarify documentation and verify what I say by adding tests. This documents has_active_dofs() in parallel.
Move declarations in DoFHandler from protected to private.

DoFHandler still had some of its member variables and functions in a protected
section from back when we still had MGDoFHandler derive from it in a meaningful
way. This is now no longer necessary, so remove the protected section and instead
sort things in some sort of natural way into the private section.
Merge pull request #161 from bangerth/private-protected-cleanup

Move declarations in DoFHandler from protected to private.
DoFHandler still had some of its member variables and functions in a protected
section from back when we still had MGDoFHandler derive from it in a meaningful
way. This is now no longer necessary, so remove the protected section and instead
sort things in some sort of natural way into the private section.
Remove unnecessary #include.
Merge pull request #162 from bangerth/remove-unnecessary-include

Remove unnecessary include
Updates to various places of the documentation.
Test lac/SchurMatrix
Test lac/SchurMatrix; Indented
Merge pull request #165 from dakshinai/test_schur_matrix

Add tests for class SchurMatrix
Adjust based on Guido's feedback.
Merge pull request #164 from bangerth/doc-fixes-8

Updates to the documentation of the quadrature formulas.