Open multiple datasets at once from `DatasetToChunks`.

[alxmrs]

Here is an initial implementation of #68.

[shoyer]

One other thing I'll note is that I've been formatting code in Xarray-Beam with the (Google internal) version of the "pyink" formatter, using 2 space indentation: https://github.com/google/pyink

It would be nice if you could apply that manually to this change -- ideally we would add some sort of automatic formatter checker.

[alxmrs]

As a small aside: I usually try to use the "commit suggestion" feature when I can. I didn't for these changes because I did them on an airplane yesterday :)

[shoyer]

@alxmrs please merge in main in order to test this

[shoyer]

@alxmrs there's a failing unit test -- can you fix it?

[shoyer]

LGTM to me (pending the formatter question).

Remember we need to merge this via the internal Google tool instead of the big green button on GitHub :)