[release] fixes a few unused parameter warnings

This is part of the patch provided by Rene in Flyspray 1355.
[release] Fixes unused parameter warnings.

This the customized rest of Rene's patch. To make doxygen happy
we use DUNE_UNUSED_PARAMETER.
[release] Removes the rest of the unused parameter warnings
Fixes compilation of overlappingschwarz.hh

Since revision 8df1d996ded6754ca47f7cb499bc1a7511e7a7bb overlappingschwarz
with UmfPack was broken due to renaming an internal variable accessed by a friend.
This patch fixes this by adding a new private method to access the internal matrix
of solver and uses that for the assembly.