Updated Documentation. Again, in the Introduction section of the step-20 tutorial a class InverseMatrix appears. Now its changed to use IterativeInverse<..> in all places to match with the code.
Minor cleanups.

Specifically: Make the FiniteElement::clone() function public, as it is of
general use. Also: Move several member variables from private to protected
as these are generally variables that derived classes are supposed to
initialize; this allows to remove rather random friend declarations for
these derived classes.
Merge pull request #152 from bangerth/cleanups

Minor cleanups. Make the FiniteElement::clone() function public, as it is of
general use. Also: Move several member variables from private to protected
as these are generally variables that derived classes are supposed to
initialize; this allows to remove rather random friend declarations for
these derived classes.
Merge pull request #155 from koecher/PR-Update-Step-20

Updated Documentation for step 20.
Clarify documentation and verify what I say by adding tests.
Fix compilation of #146 without MPI
Merge pull request #159 from kronbichler/master

Fix compilation of #146 without MPI. This closes #158.
Add Threads::Task::joinable().

The function verifies that one can call Threads::Task::join() in the
same way as one can do with std::thread::joinable() in C++11.
Add test.
Merge pull request #157 from bangerth/add-task-joinable

Add Threads::Task::joinable()
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.