cf.Field.collapse should delete a vertical CRS that spans a collapse axis

[davidhassell]

When cf.Field.collapse operates along one or more axes spanned by a vertical coordinate reference system (such as standard_name:atmosphere_hybrid_height_coordinate), the code currently deletes any domain ancillaries that span the any of the collapse axes, but leaves other domain ancillaries behind, as well as the vertical coordinate reference system.

However, the CF conventions say that missing terms should be assumed to be zero, and so if the non-parametric vertical coordinates are computed they will likely give inappropriate, but possibly plausible, results - the worst kind of error!

So, removing the CRS and its contained constructs is certainly the safe thing to do by default, but you might want to keep it, perhaps to reinstate a new correct domain ancillary. Therefore, it should be possible to override the removal of the vertical CRS with a keyword parameter to cf.Field.collapse.

PR to follow.

>>> import cf
>>> cf.environment(paths=False)
Platform: Linux-5.15.0-60-generic-x86_64-with-glibc2.35
HDF5 library: 1.12.1
netcdf library: 4.8.1
udunits2 library: /home/david/miniconda3/lib/libudunits2.so.0
ESMF: 8.2.0
Python: 3.10.8
dask: 2022.12.1
netCDF4: 1.6.0
psutil: 5.9.0
packaging: 22.0
numpy: 1.23.5
scipy: 1.9.3
matplotlib: 3.6.2
cftime: 1.6.2
cfunits: 3.3.5
cfplot: 3.1.31
cfdm: 1.10.0.2
cf: 3.14.0