Add grid_bounds_to_polygons

[dcherian]

• Add to api.rst
• Add test

This converts "bounds" in the 2-element form to an array of shapely polygons, one per grid cell. A step towards nicely packaging conservative regridding for the ecosystem. It could easily be generalized to the 4-element "vertex" form

Modified from
https://notebooksharing.space/view/c6c1f3a7d0c260724115eaa2bf78f3738b275f7f633c1558639e7bbd75b31456#displayOptions=

cc @martinfleis @benbovy @aulemahal for awareness. I'm not sure if this is at all useful for xvec but on the off-chance that it is..

[codecov]

Codecov Report

Attention: Patch coverage is 10.34483% with 26 lines in your changes missing coverage. Please review.

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Files	Patch %	Lines
cf_xarray/geometry.py	7.14%	26 Missing ⚠️

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  Lines        2364     2799     +435     
  Branches      183      208      +25     
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- Partials       33       51      +18     

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[martinfleis]

In Xvec we could flatten the array and use it as an index instead of latitude and longitude coordinates if there's a use case for that.

[mathause]

That looks very promising also for regionmask as we discussed in regionmask/regionmask#499. Is it a general assumption in cf_xarray that your grid is lat/ lon in degree?

[mathause]

I think the reason for this is that most geographic data is given in WGS84 which is -180...180 (I may be wrong here). So if you want to compare it to regional polygons it is probably a good idea to wrap the data. Still, it may be confusing to users.

Maybe this could be optional. However, I am not sure what a good name is (In regionmask I call it wrap_lon with the options 180 (= what you do here), 360 and False. That works but there may be better names).