This commit adds logic and structures for bringing in constant flux
aquifer objects-keyword AQUFLUX-from the restart file and forming
the requisite dynamic objects in the Schedule.
In particular, the 'RstAquifer' gets a new bare-bones representation
of these aquifer types and we add new constant flux aquifer objects
to the pertinent 'ScheduleState' when these exist. We also add this
aquifer type to 'data::Aquifers' in preparation of reinitialising
the simulator's aquifer container from the restart file. In
particular, this needs the total produced volume from the aquifer so
far.