Propagate UnstructuredGrid to the EclipseWriter ctor

The writer will need to know which cells are the active cells after
post-processing (because these are the cells there is stored results
for in the pressure and saturation arrays), and thus not only the
raw input grid (to get the COORD and ZCORN arrays which is not easily
detainable from the UnstructuredGrid), *and* the UnstructuredGrid
needs to be available.
Remove tucked away reference to parser in Sim.outp.
Store reference to UnstructuredGrid in EclipseWriter
Use processed grid to figure out active cell map