WIP: Add option for creating inset SkewTs

[jthielen]

Description Of Changes

As suggested in https://github.com/Unidata/MetPy/issues/1706#issuecomment-778533177, this PR adds a classmethod to SkewT to allow easier creation of an inset SkewT axes following the approach of Axes.inset_axes.

Currently just a proof of concept, I haven't tested it out yet. Just thought I'd get the prospective implementation up for any early sanity check review.

Checklist

• [ ] Closes #xxxx
• [ ] Tests added
• [ ] Fully documented
• [x] Properly acknowledge reused code from matplotlib

[dopplershift]

This approach worked better than I would have feared and is in general a 👍 , assuming it accomplishes all that's needed for doing the inset plotting.