River/Drainage checks while running imod_coupler

[Huite]

Generally, we would want to get the pointer to the river/drainage(/gbh) variables once, then just use them for the rest of the simulation.

However, MODFLOW 6 allows a certain level of dynamism, where the number of the active boundaries may change over time. The maximum number of boundaries is given by MAXBOUND, which determines the amount of memory that is allocated. However, per stress period, the number of boundaries may change, depending on the number of rows in the input. This number is stored in NBOUND.

In general, we do not expect the NBOUND to change over time: the coupler does not support this, because the exchange weights are static. This also means that the NODELIST shouldn't change over time. We could check the NODELIST after every stress period, but this is relatively expensive. A pragmatic change might be to just check the NBOUND variable.