Add xesmf and xskillscore to environment

[jthielen]

Regridding (with xesmf) and verification metrics (with xskillscore) were planned as a part of the Advanced Model Output vs. Observations project idea, so this adds those two packages to the environment list. xesmf (with ESMF) is a fairly hefty dependency, so hopefully that doesn't cause too many issues. At least v0.5.0 of xesmf is required so that we get automatic lat/lon coordinate identification.

[dopplershift]

Let's get #52 merged and try to rebase this on top--maybe that will allow the windows environment to resolve?

[jthielen]

@dopplershift Doesn't look like that worked. But, with that environment update, the error is a bit easier to read, and it looks like (for whatever reason) we can't simulatenously have xesmf and xskillscore up-to-date on windows (xesmf=0.5.0 pins xarray to <= 0.16.0, whereas xskillscore requires at least 0.16.2).

I think the fix may have to petition xesmf for a 0.5.1 release working on 0.16.2 (since it was a 0.16.1 bug that was the reason for the pin).

[dopplershift]

You can try to make the argument on the xesmf conda-forge feedstock.

[jthielen]

Closed in favor of #94, since I don't think we'll be resolving the upstream issue in time.