Grouper limited to datetime properties and coordinates

[aulemahal]

Current sdba.Grouper is limited to datetime properties like 'time.month', 'time.dayofyear'. Passing the name of an integer coordinate doesn't work.

Originally posted by @jamaa in #1735

Datetime-assumptions issues:

• Grouper(dim_name) doesn't actually group on values of the dim, it groups all values of the dim together : like Group('time') does. But it is kinda unexpected with integer dimensions.
• Grouper('time.<aux coord>') should work when the time grouping is not one of the classical "day of year", "month" or "week", but rather a custom provided integer auxiliary coord.
• The adjustment input checks will fail if the grouping dimension does not have a calendar. Which means only datetime coordinates are usable.