should guess_coord_axis optionally create a variable for unindexed coordinates

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Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
45/100
Issue type
Feature
Clarity
Mostly clear
Activity status
Stale
Tech stack
python
Domain
data

Research direction

Start at the .cf.guess_coord_axis entry point and inspect how unindexed dimensions are currently handled. Add an opt-in behavior for creating coordinate variables from dimension sizes, then verify that the resulting x and y coordinates are assigned the X and Y axes appropriately.

Written by the indexing model from the issue text.

Description

enhancement good first issue 
da = xr.DataArray(np.ones(2,3), dims=("x", "y"))

This kind of dataarray exists in CMIP datasets. x,y have absolutely no data (no values, no attrs) associated with them, and so guess_coord_axis does not do anything.

We could have .cf.guess_coord_axis(add_indexes=True) that effectively does

da["x"] = np.arange(da.sizes["x"])
da["y"] = np.arange(da.sizes["y"])

# now do the guessing

This will assign axis='X' and axis='Y' appropriately. I think this would be an OK opt-in feature, though we need a better name than add_indexes.

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Python
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