Add coordinate transform index

[benbovy]

Adding an example for xarray.indexes.CoordinateTransformIndex.

@Cadair -- to give you some context, we are building in this repository a gallery of Xarray index examples. I included here your Astropy WCS experiment from https://gist.github.com/Cadair/4a03750868e044ac4bdd6f3a04ed7abc, it is a great illustration for CoordinateTransformIndex! I hope you don't mind?

[scottyhq]

Suggested change

	da_2d.plot.pcolormesh(x="pos_eq_ra", y="pos_eq_dec");
	da_2d.plot.pcolormesh(x="pos_eq_ra", y="pos_eq_dec", norm=colors.LogNorm());

The figure looked like no data to me (granted I'm in a bright sunny place right now :)

[benbovy]

Yeah it's a dark sky! :)

I've improved the contrast a bit and changed the colormap.

[scottyhq]

This is great! Not sure if it's possible to 'double' list this in the TOC, because it would also be nice to have an Astronomy section to go along with Earth Sciences to encourage more non geoscience examples and participation

[benbovy]

Not sure if it's possible to 'double' list this in the TOC, because it would also be nice to have an Astronomy section to go along with Earth Sciences to encourage more non geoscience examples and participation

Yes I agree that would be nice. It'll probably be easier if this is eventually implemented in a Python library (or in Astropy itself!). In the meantime, I mentioned "Astronomy" in the title of the example subsection.

[Cadair]

Awesome! Thanks for making the example work nicely. Hopefully soon I can come back to this and make the slicing and other nice things work.