Pin xarray to unreleased master and rechunker to 0.4.2 for use with pip.

[sharkinsspatial]

Closes #111

[sharkinsspatial]

@rabernat After merging, can you create a new release as well since 0.2.0's requirements.txt contains this non-existent xarray version ad82fb6.

[rabernat]

Thanks Sean!

[rabernat]

I think we have a hung test.

[rabernat]

After merging, can you create a new release as well since 0.2.0's requirements.txt contains this non-existent xarray version ad82fb6.

I'm not sure that's what you want. If we create a new release now from master, it will also include the refactor of #101, which will probably break your test recipes. I guess we should have done this before the 0.2.0 release. 🤦

In any case, xarray 0.17.1 will be released any day now.

[sharkinsspatial]

@rabernat I've updated both pangeo-forge-prefect and my PR against staged-recipes to use the changes from #101 and run the recipe on our bakery as a test. Is there any other breaking changes in master that I have not taken into account?

But I agree it is worthwhile to hold off until we can pin the xarray version after the release of 0.17.1.