Fixes #1437 

# Description
``primod`` expects a 2D rate array or single layer array for the dummy
MODFLOW6 package, as otherwise the ``squeeze("layer")`` it does returns
an error. I think the safest for iMOD Python is to return a 2D array
with extra layer coordinate. I added some extra tests to test if this
RCH package could be clipped and regridded, as in most/all fixtures for
clipping/regridding I added packages with layer dimension. Luckily these
two extra tests turned out to be mostly for my own peace of mind, as
they ran without problems.

# Checklist
- [x] Links to correct issue
- [x] Update changelog, if changes affect users
- [x] PR title starts with ``Issue #nr``, e.g. ``Issue #737``
- [x] Unit tests were added
- [ ] **If feature added**: Added/extended example