Xarray accessor updates to remove automatic quantification and in-place operations

[jthielen]

Description Of Changes

Update of #1378 on new fork and after rebase on latest master. Original comment below:

Based on lingering discussions (particularly in #1354), this PR implements a few cleanups on the xarray accessors needed for work on #1353 to continue, namely, removing automatic conversion to quantities in parse_cf (since its loading of data into memory can cause errors) and making sure all operations are not in-place. There is one place where "side effects" still occur (caching of the coordinate parsing results in the _metpy_axis attribute), but I figured this was still okay (and hard to avoid, since it is invoked when using a property).

Checklist

• ClosesHow does cross_section recognize projections? #1354
• Tests added
• Fully documented

[jthielen]

I realized that this uses xr.Dataset.map, which was introduced in v0.14.1 (Nov 2019). Should we bump minimum version from v0.13.0 (Sep 2019), or should I have a fallback to xr.Dataset.apply, which is pending deprecation?

[dopplershift]

This looks good. I like the extensive additions in notes in the docs and comments.

[dopplershift]

Eh, what's another two months shorter on our xarray horizon? Go ahead and bump. Would just 0.14.0 work? (Could always let our tests determine that.) If not 0.14.1's fine.

Hey does that mean we can work around some of those deprecated functions now? 😄

[dopplershift]

Answered my own question here as I did some digging for other reasons. We need 0.14.1 for xr.Dataset.map, so that's fine. 0.14.1 was a weird release for xarray in that it includes new "features".

[jthielen]

Answered my own question here as I did some digging for other reasons. We need 0.14.1 for xr.Dataset.map, so that's fine. 0.14.1 was a weird release for xarray in that it includes new "features".

Yeah, xarray hasn't been the best at following semver...their "patch" releases sometimes have new APIs and breaking changes.

Hey does that mean we can work around some of those deprecated functions now? 😄

I'll take a look after this PR!

[dopplershift]

Not sure why Travis shows queued, it's passed. Codacy is...well, wrong.

[jthielen]

So now it's mostly just the updated version of #1353 to go before 1.0-rc2?

[dopplershift]

Sounds great. I promise this one will actually get my attention. Thanks for knocking these out. I'm really happy with how CI is working with these.