Adapt to and impose latest xarray

[aulemahal]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes #xyz
• Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)
• HISTORY.rst has been updated (with summary of main changes)
• bumpversion (minor / major / patch) has been called on this branch
• Tags have been pushed (git push --tags)

• What kind of change does this PR introduce?
  This pushes the xarray dependency to use the latest version, 0.16. As a consequence it allows to remove the import checks for xclim.sdba.
  Moreover, xarray 0.16 now offers xr.infer_freq, porting pd.infer_freq to cftime indexes, so I modified datachecks.check_daily to use this instead. Also, added a test and test parametrization to test with both cftime and pandas.

• Does this PR introduce a breaking change?

Yes. Moving to xarray 0.16 might break things. Let's see in the builds!

• Other information:

The History.rst entry for 0.19 has been structured in a clearer way so its easier for user to understand the many breaking changes that release will bring.

[huard]

All good.

[coveralls]

Pull Request Test Coverage Report for Build 2080

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.06%) to 91.026%

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Totals
Change from base Build 2078:	0.06%
Covered Lines:	3114
Relevant Lines:	3421

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💛 - Coveralls