Ixmp4/datetime

[danielhuppmann]

Please confirm that this PR has done the following:

• [x] Tests Added
• ~Documentation Added~
• ~Name of contributors Added to AUTHORS.rst~
• [ ] Description in RELEASE_NOTES.md Added

Adding to RELEASE_NOTES.md (remove section after adding to RELEASE_NOTES.md)

Please add a single line in the release notes similar to the following:

- (#XX)[http://link-to-pr.com] Added feature which does something

Description of PR

This PR is a first step to support writing subannual (datetime) timeseries to an ixmp4 platform.

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 95.1%. Comparing base (ddbb88e) to head (7b24f3a). :warning: Report is 56 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff           @@
##            main    #912    +/-   ##
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  Coverage   95.0%   95.1%            
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  Files         64      66     +2     
  Lines       6134    6399   +265     
======================================
+ Hits        5828    6086   +258     
- Misses       306     313     +7     

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[danielhuppmann]

Thanks @phackstock

The release notes entry seems to be missing for now

Added

An entry to the docs page about firstly the datetime upload option and the datetime formatting rules would be good.

The documentation includes an explanation of the datetime use case, see the Data Model page. The to_ixmp4() documentation doesn't make any reference to the time domain, so this PR removes an undocumented NotImplementedError - so I would not make the documentation more complex than necessary.

As codecov points out, a test with a 'time' dimension with datetime and years combined would be good.

Tried to add a test for the mixed time-domain case and realized that ixmp4 does not support that yet - for now, I added a clear NotImplementedError in this PR. I suggest to merge this PR, then I'll see if I can fix ixmp4, and then extend to pyam to_ixmp4() functionality.