Improve ixmp.model internals

[khaeru]

• Move the gams_version() utility function into ixmp (was in message_ix): https://github.com/iiasa/message_ix/blob/aa02d8c545aec4466fb98290cafbba598a9e56b8/message_ix/models.py#L192-L194
• Allow Backend.init_item() (and thus Model.initialize_items()) to accept tuples/other sequences of strings for the idx_sets and idx_names parameters—rather than only lists.

How to review

Note that the CI checks all pass.

PR checklist

• [x] Add or expand tests.
• [x] Add, expand, or update documentation.
• [x] Update release notes.

[codecov[bot]]

Codecov Report

Merging #376 into master will increase coverage by 0.0%. The diff coverage is 100.0%.

@@          Coverage Diff           @@
##           master    #376   +/-   ##
======================================
  Coverage    96.8%   96.8%           
======================================
  Files          46      46           
  Lines        5253    5281   +28     
======================================
+ Hits         5086    5114   +28     
  Misses        167     167           

Impacted Files	Coverage Δ
ixmp/backend/base.py	98.8% <ø> (ø)
ixmp/core.py	94.8% <ø> (ø)
ixmp/model/base.py	100.0% <100.0%> (ø)
ixmp/model/gams.py	100.0% <100.0%> (ø)
ixmp/tests/core/test_scenario.py	100.0% <100.0%> (ø)
ixmp/tests/test_model.py	100.0% <100.0%> (ø)
ixmp/utils.py	96.2% <100.0%> (+0.1%)	:arrow_up: