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.