Xmip

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](https://github.com/mathause/regionmask) in a new module `cmip6_preprocessing.regionmask`.

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

The pypi release was not clean.

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