Add multiplatform testing builds

[Zeitsperre]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes issue Add GitHub CI tests for macOS, Windows and Anaconda #126
• 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)

What kind of change does this PR introduce?:

• Adds macOS builds to CI testing suite

Does this PR introduce a breaking change?:

No.

Other information:

The testing data is fetched currently from two different repositories. unix-style file locking is used when gathering these files, which isn't available on Windows. Additionally, xesmf is not available on Windows either (conda).

Will try seeing if tox builds could potentially work on Windows, otherwise I'll adjust the metadata to suggest that clisops doesn't support Windows.

[coveralls]

Pull Request Test Coverage Report for Build 15886172993

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 87.164%

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Totals
Change from base Build 15885127333:	0.0%
Covered Lines:	3110
Relevant Lines:	3568

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💛 - Coveralls

[Zeitsperre]

@cehbrecht Consider this PR a WIP; I need to get access to a Windows computer to see what's up, but essentially, there should be a simple way to load UNIX-style file paths on Windows and still have everything work as intended.