What problem does your feature request solve?

CSET currently only supports a Gregorian calendar, thus we want to extend calendar support to more idealised calendars that are apart of the CF Metadata conventions.

Describe the solution you'd like

Addition of a function within constraints.py to return a PartialDateTime object, such that they can be used within generate_time_constraint to create a time constraint irrespective of calendar type.

Describe alternatives you've considered

The use of a cftime.datetime object was discussed but would have required the specification of the target calendar.