Partial reassembler didn't compile for staggered disc. This fix makes it easier to implement a reassembler for new discretizations.
If merged this should also be squashed into the main commit as otherwise staggered doesn't compile.
@bernd: FinalizeAssembly reset the vertex colors to green. This is now done at the beginning of compute colors. Is this the same? For the first iterations all dofs are red anyway and between
computeColors nothing changes in the reassembler, right?