Miscellaneous improvements for 2022-W30

[khaeru]

• Adjust to genno 1.12 and set this as the minimum required version.
• Handle a ValueError in TimeSeries.add_timeseries() / JDBCBackend.set_data(); partly addresses #296.
• Bugfix in ixmp._config.Config: config values loaded from file were squashed with defaults, if (and only if) those defaults were set downstream, e.g. in message-ix, message-ix-models, or message-data. Preserve the values, as expected.
• Improve .reporting.computations.update_ts():
  • Use TimeSeries.check_out(…, timeseries_only=True).
  • Tolerate failure to add each individual item, add optional strict argument.
• Simplify units handling in .reporting.computations.update_scenario()

How to review

• Read the diff and note that the CI checks all pass.

PR checklist

• [x] Continuous integration checks all ✅
• [x] Add or expand tests; coverage checks both ✅
• [x] Add, expand, or update documentation.
• [x] Update release notes.

[codecov[bot]]

Codecov Report

Merging #451 (d22f5ec) into main (e795853) will increase coverage by 0.0%. The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #451   +/-   ##
=====================================
  Coverage   98.6%   98.6%           
=====================================
  Files         42      42           
  Lines       4446    4460   +14     
=====================================
+ Hits        4384    4398   +14     
  Misses        62      62           

Impacted Files	Coverage Δ
ixmp/tests/reporting/test_reporter.py	100.0% <ø> (ø)
ixmp/_config.py	96.7% <100.0%> (-0.1%)	:arrow_down:
ixmp/backend/jdbc.py	95.2% <100.0%> (+<0.1%)	:arrow_up:
ixmp/model/base.py	100.0% <100.0%> (ø)
ixmp/reporting/computations.py	100.0% <100.0%> (ø)
ixmp/reporting/reporter.py	100.0% <100.0%> (ø)
ixmp/tests/reporting/test_computations.py	100.0% <100.0%> (ø)