Rationalise netCDF datetime code

[jonseddon]

At the moment some code uses netcdftime.datetime and others use cf_units.datetime. netcdftime has been replaced by cftime (https://pypi.org/project/cftime/). cf_units is a pain to install as it requires udunits for the operating system and its calls could also be replaced with cftime. cftime doesn't provide CALENDAR_GREGORIAN, etc. constants and so some refactoring would be required.