"double" and "single" keywords to cf.write do not change the output data types #65
Description
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