Modernize UMFPackSolver
I want to make UMFPackSolver accept matrices of type MultiTypeBlockMatrix. The hurdle is colcompmatrix.hh, which somehow funnels the matrix data into a rudimentary scalar compressed-column matrix, which then somehow ends up being handed to UMFPack. This MR tries to do a bit of cleanup/modernization, in the hope that the code will become easier to maintain and generalize.
The main part of the cleanup is that the above-mentioned compressed-column matrix turns into a real dune-istl matrix, or at least something close to it.