"double" and "single" keywords to cf.write do not change the output data types

These keywords are present, but have no effect. The code to implement them was inadvertently removed at version 3.0.0

>>> cf.environment(paths=False)
Platform: Linux-5.3.0-46-generic-x86_64-with-debian-buster-sid
HDF5 library: 1.10.4
netcdf library: 4.6.3
udunits2 library: /home/david/anaconda3/lib/libudunits2.so.0
python: 3.7.3
netCDF4: 1.5.3
cftime: 1.1.1
numpy: 1.18.1
psutil: 5.4.7
scipy: 1.4.1
matplotlib: 3.2.0
ESMF: 8.0.0
cfdm: 1.8.1
cfunits: 3.2.5
cfplot: not available
cf: 3.3.0