Add unit- and axis-aware selection to the metpy accessors

[jthielen]

This PR adds .loc[] and .sel() to both the DataArray and Dataset accessors to allow for unit-aware indexing/selection. Also, for DataArrays (for which axis types are uniquely determined), I also added in selection via the axis name ('time', 'vertical', etc.).

As an example, now something like this can be done:

temperature.metpy.sel(vertical=500 * units.hPa)

Closes #970.

Also, I reused some code from xarray with modification for this (the loc indexers). What is the proper way to cite this in the code given their license?

[dopplershift]

Instead of copying in xarray code, can we just delegate to the loc attribute? It'd be nice to keep the licensing simpler, plus it would lessen the burden of updating with future changes to xarray.

[jthielen]

While I couldn't think of a way to not reuse the _LocIndexer class approach, I've changed the internals of the classes to delegate more to the base .loc attributes rather than parallel the xarray code in going to the .sel method. If this approach is better, I can squash the commits and get rid of the old copy-and-modify approach. Otherwise, let me know what else I could try!

[dopplershift]

I still need to give this a more detailed review, but I definitely prefer this to the copy-and-modify version. Thanks!

[dopplershift]

Nice work. The xarray handling code is getting a bit big, but not too unwieldy...yet. We may yet be broken by xarray because we're using things they consider outside their public API, but that's ok. Small price to pay for this nice bit of functionality.

[dopplershift]

If you can rebase this to merge cleanly, I think we're good to go here.

[jrleeman]

Rebased and clean!