cf.weighted doesn't work with DataArrays

[malmans2]

Looks like .cf.weighted breaks with DataArrays.

from cf_xarray.datasets import airds
weights = xr.ones_like(airds["air"])
weights.attrs = {}
airds.cf.weighted(weights)

ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

Note that I can't do this, because only Datasets and DataArrays have the .cf accessor:

airds.weighted(weights).cf.mean("T")

AttributeError: 'DatasetWeighted' object has no attribute 'cf'

One possible workaround is:

airds.weighted(weights).mean(airds.cf.axes["T"])

or

airds = airds.assign_coords(weights=weights)
airds["air"].attrs["cell_measures"] += " weights: weights"
airds.cf.weighted("weights").mean("T")

but I don't think that's what we want.

[dcherian]

airds.cf.weighted(weights)

I think we could fix this in the mapper. Pretty sure we only test airds.cf.weighted('area')