Modflow 6 well edge case when located on partition border

[JoerivanEngelen]

When Well's are located on the edge of a partition border, wells are located to both partitions for unstructured grids, and since #1189 also for stuctured grids.

After a discussion with @Huite, I concluded that the easiest way forward is probably to add a post-processing step to Modflow6Simulation.split, which looks for duplicate well entries (maybe by index or 'id' if possible) amongst different model partitions and select one.

Note: Wells are still grid agnostic after splitting, assigning wells to cells occurs in the call to Modflow6Simulation.write().