Integration with Pandera

This issues simply cross references the integration of xarray-schema within [Pandera](https://pandera.readthedocs.io/en/stable/). See https://github.com/pandera-dev/pandera/issues/705 for progress there. While we may continue to develop functionality here, long term goal to migrate to Pandera when xarray support is added. 

xref: https://github.com/pandera-dev/pandera/issues/705