ecfv: syncronize the values of overlap and ghost elements from their master process prior to assembling

the algebraic overlap stuff still has a few bugs which prevent the
ECFV discretization from working in parallel mode. (these will be
fixed soon.)
algebraic overlaps: some minor renaming
algebraic overlap: handle the case where no entity which is directly on the process border exists

for example this happens for element based methods. (elements are not
fully contained within the border.)
algebraic overlap: make it work with element based methods

i.e. the element centered finite volume scheme is now fully MPI
parallel. (the only catch is that the grid must support an overlap of
size one or ghost elements. AFAICS this is the case for all parallel
DUNE grids, though.)