Regridding: handling of zonal mean datasets and datasets with shifted longitude frame

[sol1105]

Pull Request Checklist:

• [x] This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes issue #217 for the regrid operator
  • The part dealing with zonal mean datasets is not an open issue.
• [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?:

• Remapping tests for zonal mean datasets
• Remapping tests for cordex datasets
• Longitude frame fix (#218) now also applied for remapping
• Remapping tests for shifted longitude frame.

Does this PR introduce a breaking change?:

No

Other information:

• Further tests required regarding longitude frame fix for remapping

[coveralls]

Pull Request Test Coverage Report for Build 7277424374

• 7 of 7 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.4%) to 72.62%

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Totals
Change from base Build 7051697572:	0.4%
Covered Lines:	1724
Relevant Lines:	2374

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💛 - Coveralls