xgcm vertical coordinate interpolation

[rabernat]

Thanks for maintaining this super cool and useful project.

We recently released a new version of xgcm which supports efficient vertical coordinate interpolation (implemented in numba) for xarray datasets: https://xgcm.readthedocs.io/en/latest/transform.html

It would be interesting to try the isentropic interpolation benchmark with this. @jbusecke could maybe help out with this.

[jbusecke]

Whoa, this is so cool! I'd be thrilled to put xgcm to the test!

[jbusecke]

See #2 for an initial attempt.