update transform_overseas when depts are already known

For instance in the example_rp_logement_2017 example, you end up with a "insee_dep" column.

For now, transform_overseas cannot let you choose a column of INSEE dpts codes and simply perform a merge to retrieve dpts geometries.

This ends up with (in the same example) a 49.138 iterations long loop to perform transform_overseas which might not be needed.

This needs to be thoroughly checked : I just ran onto this while updating the examples and opened the issue to keeps this in mind.