This commit adds a new, focused, member function
NumericalAquifers::allAquiferCellIds()
which returns a vector of those Cartesian/global cells that have
been marked as defining the model's numerical aquifers through the
AQUNUM keyword. We intend to use this to identify those NNCs that
go to numerical aquifers--or between numerical aquifer cells--as
those may need special treatment when processing the MULTREGT
keyword.
b84c5f46bf