Previously, the preconditioners using matrices, derived privately from

Previously, the preconditioners using matrices, derived privately from
PreconditionRelaxation, but introduced the only function of the base
class later on explicitely into the public area. Since private
inheritance interferes with the use of the Subscriptor class in the
base class (we can't cast to it any more), change the inheritance to
public and remove the explicit "using" declarations.

git-svn-id: https://svn.dealii.org/trunk@4892 0785d39b-7218-0410-832d-ea1e28bc413d