The calendar CF property.
This property is a mirror of the calendar stored in the Units attribute.
Examples
>>> c.calendar = 'noleap'
>>> c.calendar
'noleap'
>>> del c.calendar
>>> f.setprop('calendar', 'proleptic_gregorian')
>>> f.getprop('calendar')
'proleptic_gregorian'
>>> f.delprop('calendar')