[Bug] iMOD5 Wel import: Duplicated entries within IPF are not summed in steady-state

[JoerivanEngelen]

We conduct a groupby by x, y, id, filter_top and filter_bottom. There unfortunately are cases where this results in non-unique groupby's and hence rates should be summed. This is at least going wrong in steady-state:

# imod.mf6.wel.py
...
def _df_groups_to_da_rates(
    unique_well_groups: pd.api.typing.DataFrameGroupBy,
) -> xr.DataArray:
    # Convert dataframes all groups to DataArrays
    is_steady_state = "time" not in unique_well_groups[0].columns
    if is_steady_state:
        da_groups = [
            xr.DataArray(df_group["rate"].iloc[0]) for df_group in unique_well_groups
        ]
...

The .iloc[0] selects the first entry in this group (Assuming there is always only one entry). Instead a call to sum() should be made.