Implement a rex backend for xarray (rexarray)

[ppinchuk]

Add a backend for xarray that allows users to read in rex-style NREL data.

The implementation itself closely follows the h5netcdf backend implementation:
https://github.com/pydata/xarray/blob/main/xarray/backends/h5netcdf_.py

Lazy loading is fully supported, which is a big reason why this implementation is so long.

HSDS and S3 (via fsspec) access is explicitly supported.

[bnb32]

Will take a deeper dive later but here are just a couple random comments.

[ppinchuk]

@bnb32, @grantbuster, @castelao Friendly bump :)

Curious to hear if anyone has tried using this and/or had success with it

[castelao]

@ppinchuk , some minor comments.

[bnb32]

@bnb32, @grantbuster, @castelao Friendly bump :)

Curious to hear if anyone has tried using this and/or had success with it

Digging back in is on the list for next week :)

[bnb32]

Overall, this looks great to me. I messed around in a notebook a little bit and things worked well, including within calls to the sup3r Loader objects. A couple random comments added. Main question on the use of time_index vs time.

[bnb32]

Do you find that using slots here reduces mem use significantly? This is one of those things I'm aware of but rarely use.

[ppinchuk]

I used __slots__ quite heavily in my grad work, but not so much at NREL. IIRC the guidance is that it can be beneficial if you intend to have thousands of instances of the object. Not sure if that actually applies here, but this is what the xarray developers did for their array wrappers and I figured they had a good reason for doing so (so I copied them😄).

[grantbuster]

Dare I say this is ready for review? :P

One question for all, but especially @grantbuster : should we make fsspec a base rex dependency? I think this library is used all over the place, including in our other dependencies like pandas. This would mean users can immediately access S3 files without having to specify [s3] during installation. Did we make it an optional dependency intentionally? I know we had to make h5pyd optional because of compatibility issues with the wtk code

Fine by me to add fsspec as a default req!