Fix id error in wq wells

Joeri van Engelenrequested to merge
issue_357_id_error_wells into master

Fix #357 (closed)

This error occurred when wells without a time dimension were assigned to an iMOD WQ model, but had a layer dimension. In that case a column with "id" was removed from the dataframe, despite it being required downstream.

I also expanded the two missing tests to have a full coverage.

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