More Tests and masking functionality

This release introduces some small improvements, supporting the newests xarray version (v0.15.1) and adding unit tests for the preprocessing module (Thanks to @aaronspring for helping with this).

It also adds masking functionality based on regionmask in a new module cmip6_preprocessing.regionmask.

More supported models

The model database now includes all models with ocean output in the pangeo cloud.

Fixing some release growing pains

The pypi release was not clean.

Initial release

This is the initial release of cmip6_preprocessing. Initial testing was successful and is described in notebooks/tutorial.ipynb.