Improve performance of create_mask

[aulemahal]

Pull Request Checklist:

• [x] This PR addresses an already opened issue (for bug fixes / features)

  • This PR fixes #160

• [x] Tests for the changes have been added (for bug fixes / features)

• [x] Documentation has been added / updated (for bug fixes / features)

• [x] HISTORY.rst has been updated (with summary of main changes)

• [x] I have added my relevant user information to AUTHORS.md

• What kind of change does this PR introduce?: Adds PyGEOS as an optional dependency. In create_mask, when it imported the intersection between the points and the current polygon is computed using its covers function, which is equivalent to touches+contains, but with the same performance as contains alone.

• Does this PR introduce a breaking change?: No.

• Other information: I also modified the tests in ops/test_subset.py so that they pass with cftime==1.5.0.

[agstephens]

@aulemahal this looks good to me. @ellesmith88 is on leave this week. @ellesmith88 please review and merge when you are back. Thanks