Add to_ixmp4() method

[danielhuppmann]

Please confirm that this PR has done the following:

• Tests Added
• Documentation Added
• Name of contributors Added to AUTHORS.rst
• Description in RELEASE_NOTES.md Added

Description of PR

This PR adds a method to_ixmp4() to easily save all scenarios in an IamDataFrame (including meta indicators) to an ixmp4 platform database instance.

The method name to_ixmp4() doesn't seem elegant, any alternative suggestions of comments?

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (98cb3ed) 94.6% compared to head (0da2f88) 94.8%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
pyam/ixmp4.py	94.4%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #797     +/-   ##
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+ Coverage   94.6%   94.8%   +0.1%     
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  Files         62      64      +2     
  Lines       6040    6089     +49     
=======================================
+ Hits        5719    5775     +56     
+ Misses       321     314      -7     

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[phackstock]

Looks good to me.
Just one potential issues that I'd see.
What if an error occurs between lines 2347 and 2351?
This way we could end up in a situation where a run is created and no data are added.
Or data are added but the run isn't set as default.
@meksor is more qualified to speak on this but I wonder if it's possible to wrap this entire operation in a try...except clause and roll back the changes in the data base in case anything goes wrong.

[danielhuppmann]

Nice catch, @phackstock. A roll-back is not possible in ixmp4 (because the creation of a run and adding of a run are separate actions), and there is no possibility (yet) to delete a run., but I'll add a to-do.

[phackstock]

Ah I see, then the quick fix might be to still use a try except and delete the whole run if we encounter an error. Just to avoid having half finished ghost runs.

[danielhuppmann]

Added an issue to delete runs in ixmp4, see iiasa/ixmp4#29.

Added a quickfix and tests to at least test for the obvious failures...

Once this PR is merged, I'll add a method read_ixmp4().

[phackstock]

Looks good to me in principle.
Some minor comments and questions in line.