Updates to subset and coordinate detection

[sol1105]

Pull Request Checklist:

• [x] This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes issue #230
• [x] Tests for the changes have been added (for bug fixes / features)
• [ ] 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?:

• The subset operator for time components fails for example when selecting day 1-31 of a dataset with 360day calendar. This issue is related to #230. This should be fixed with this PR.
• Some CMIP5/6 datasets feature incorrect units attributes for rotated latitude/longitude coordinates (degrees_east/north instead of degrees) and confuse cf-xarray. With this PR, coordinates are identified first by cf-xarray as usual, and if this fails, they are identified by standard_name.

Does this PR introduce a breaking change?:

No.

Other information:

[coveralls]

Pull Request Test Coverage Report for Build 7447890195

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 72.477%

---

Totals
Change from base Build 7398712606:	0.1%
Covered Lines:	1738
Relevant Lines:	2398

---

💛 - Coveralls