Use cftime library for model calendars

[anton-seaice]

There is a confusing mix of cftime datetime objects and standard python datetime objects used through Payu.

e.g. cftime:

payu/payu/models/cesm_cmeps.py

Line 383 in 477a5f7

return cftime.datetime(

payu/payu/models/model.py

Line 522 in 477a5f7

cftime.datetime object

and datetime:

payu/payu/models/um.py

Line 302 in 477a5f7

return datetime.datetime(year=d[0], month=d[1], day=d[2],

payu/payu/models/cice.py

Line 212 in 477a5f7

init_date = datetime.date(year=setup_nml['year_init'], month=1, day=1)

Python datetime objects only believe in the Gregorian calendar (although the implementation appears more like a proleptic_gregorian ? )

Payu even has functions to make python datetimes pretend to understand other calendars , e.g. :

payu/payu/calendar.py

Line 44 in 477a5f7

def date_plus_seconds(init_date, seconds, caltype):

We could (possibly should) move to using cftime everywhere in payu for model time, and only use python datetimes for real world time (e.g. machine time).

Related: #305