Add AR6 regions

Pull request

Add AR6 regions using regionmask, reference notebook is here https://github.com/SantanderMetGroup/ATLAS/blob/master/reference-regions/notebooks/Reference_Regions_Python.ipynb

Please confirm that this pull request has done the following:

• Tests added
• Documentation added (where applicable)
• Example added (either to an existing notebook or as a new notebook, where applicable)
• Description in CHANGELOG.rst added

Adding to CHANGELOG.rst

Please add a single line in the changelog notes similar to one of the following:

- (`!XX <https://gitlab.com/netcdf-scm/netcdf-scm/merge_requests/XX>`_) Added feature which does something
- (`!XX <https://gitlab.com/netcdf-scm/netcdf-scm/merge_requests/XX>`_) Fixed bug identified in (`#YY <https://gitlab.com/netcdf-scm/netcdf-scm/issues/YY>`_)