Add diagnostics.health_checks

[RondeauG]

Pull Request Checklist:

• [x] This PR addresses an already opened issue (for bug fixes / features)
  • No.
• [x] (If applicable) Documentation has been added / updated (for bug fixes / features).
• [x] (If applicable) Tests have been added.
• [x] This PR does not seem to break the templates.
• [x] HISTORY.rst has been updated (with summary of main changes).
  • [x] Link to issue (:issue:number) and pull request (:pull:number) has been added.

What kind of change does this PR introduce?

• New function diagnostics.health_checks that performs a bunch of checkups on a dataset.
  • You have full control on which checkup should return a warning and which should raise and Exception.
  • By default, the function returns nothing. However, you can set return_flags=True to retrieve the dataset created by xclim's data_flags (as long as the function does not raise an Exception elsewhere).
• Added 2D lon-lat and grid_mapping to datablock_3d when the grid is not regular.

Does this PR introduce a breaking change?

• Removed diagnostics.fix_unphysical_values, although I seriously doubt that anyone was using it.

Other information:

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[RondeauG]

I should have answered all comments. Did you want to check again before we merge?