when we try to send data, it might happen, that for some reason we don't want to any data.
This happens, when the dynamic size of our CommPolicy is 0. In that case we must not add a
MessageInformation, otherwise we get an MPI error.

VariableSizeCommunicator now appears in doxyen and the parallel
index sets are a subsection of the parallel section.

This patch introduces a new macro DUNE_UNUSED_PARAMETER(parm) for marking
unused parameters that are there for a purpose (e.g. the prescribed interfac
The macro simply performs a cast to void. In addition all unused parameter w
that I found were removed.

[[Imported from SVN: r6630]]

[[Imported from SVN: r6628]]

[[Imported from SVN: r6264]]

[[Imported from SVN: r6259]]