Well.from_imod5_data: Compute average well rates for "steady state" model simulations

[JoerivanEngelen]

The iMOD5 documentation writes that iMOD5 computes an average for steady-state well rate timeseries when a steady-state simulation is written. In my steady-state LHM simulation conversion test, I found that this results in quite different rates than just picking some rates at a timestep, as extraction rates vary through time a lot.

We currently do not have a clear API to generate steady state simulations (see #1308 ). It is crucial for testing the LHM, however, that we have some functionality to compute average well rate timeseries, instead of resampling these timeseries.