Add missing formatting words and operations for generic stats

[aulemahal]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes an issue raised by chat by @RondeauG
• Tests for the changes have been added (for bug fixes / features
  • (If applicable) Documentation has been added / updated (for bug fixes / features)
• CHANGES.rst has been updated (with summary of main changes)
  • Link to issue (:issue:number) and pull request (:pull:number) has been added

What kind of change does this PR introduce?

• Adds "integral", "count", "doymin" and "doymax" to the formattable words known by xclim. This would allow customizing long names and description. This mostly touches generic.stats for the moment.
  • I couldn't find how to word "count" and "doym??" as adjective...
• I realized doymin was not accessible to generic.stats, so I added a mechanism to do it.

Does this PR introduce a breaking change?

No. It kinda only removes a warning for now : When op='integral' in generic.stats, the previous code raised a warning and then puts "integral"... which is what my added mapping does too.

Other information: