[doc] Improves doxygen documentation of schwarz.hh.

Namely we try to document the template parameters in more detail and
make sure that cross reference to OwnerOverlapCopyCommunication,
vectors, and matrix implemetation appear.
Improve output given by UMFPack through InverseOperatorResult

The elapsed wallclock time is readily available by UMFPack
and can be written to InverseOperatorResult.elapsed!