check whether initialized calendar type equals calendar type of new file added by add_observations() and add_uninitialized()

https://github.com/bradyrx/climpred/pull/294 currently converts all non xr.CFTimeIndex objects into cftime.ProlepticGregorian. This is problematic for instance if the model being used is on a noleap or 360day calendar.

The ideal case would be to encourage users to handle the cftime stuff on pre-processing. But we can also look for calendar attributes and so on to infer the calendar before converting.

I have a useful set of calendar-related functions here: https://github.com/bradyrx/esmtools/blob/master/esmtools/temporal.py

[aaronspring]

basically we infer calendars now. PM uses Leap/NoLeap, hindcast Gregorian. Maybe we add a config.yaml with these defaults and a user that wants to change this can change it here.

[bradyrx]

Yes. For instance this came up in https://github.com/pangeo-data/climpred/issues/393. In the current system you could add a hindcast with calendar DatetimeNoLeap. then maybe you pull in observations with DatetimeGregorian. It would work but then alignment breaks bc xarray sees them as two different datetimes. So the error would just tell users to update the calendar, since calendar switching is not easy to automate.