Add Unidata CDM conventions to coordinate_criteria

[dcherian]

From @kthyng in #315

The following looks like some older convention. We could easily support this in

cf-xarray/cf_xarray/criteria.py

Line 15 in 7e66367

coordinate_criteria: MutableMapping[str, MutableMapping[str, Tuple]] = {

		"_CoordinateTransformType": "Vertical",
        "_CoordinateAxisType": "GeoZ",
        "_CoordinateZisPositive": "down",
        "_CoordinateAxes": "sigma",

[kthyng]

@dcherian Yep good point!

[dcherian]

This stuff is from the Unidata CDM: https://docs.unidata.ucar.edu/netcdf-java/current/userguide/coord_attr_conv.html .