External cell measure netCDF variable names are not always preserved by `cf.write`

[davidhassell]

When a writing multiple fields that have external cell measures, the the external variable names in the output file are appended with numbers to make them unique. However, they should be preserved as they are.

>>> import cfdm
>>> cfdm.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
Python: 3.10.8
netCDF4: 1.6.0
numpy: 1.23.5
cfdm.core: 1.10.0.2
cftime: 1.6.2
netcdf_flattener: 1.2.0
cfdm: 1.10.0.2