Add TimeSeries.transact(); clean up code tree

[khaeru]

• Add a new method, TimeSeries.transact().
• Deprecate .utils.isscalar(). This is not used within ixmp, only in message_ix; the Numpy method should be used instead.
• Add a “Data model” page to the documentation, based on the (private) wiki page of the same name.
• Update genno requirement to >= 1.6.0 (cf. Adjust for genno 1.6, pyam 1.0 message_ix#492).
• Split the (large) ixmp.core file into .core.platform, .core.timeseries, and .core.scenario.
  • Move methods .from_url and .*_meta from Scenario to TimeSeries, since none of these are specific to model data.
• Collect Java binaries related to the JDBCBackend under ixmp/backend/jdbc/.
• Improve test coverage and typing.

PR checklist

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

[codecov]

Codecov Report

Merging #422 (b700468) into main (59f3a1d) will increase coverage by 0.5%.
The diff coverage is 98.8%.

@@           Coverage Diff           @@
##            main    #422     +/-   ##
=======================================
+ Coverage   97.9%   98.5%   +0.5%     
=======================================
  Files         39      41      +2     
  Lines       4236    4321     +85     
=======================================
+ Hits        4148    4257    +109     
+ Misses        88      64     -24     

Impacted Files	Coverage Δ
ixmp/cli.py	100.0% <ø> (+1.1%)	⬆️
ixmp/testing/jupyter.py	100.0% <ø> (ø)
ixmp/tests/core/test_meta.py	100.0% <ø> (ø)
ixmp/core/scenario.py	97.9% <97.9%> (ø)
ixmp/core/timeseries.py	98.6% <98.6%> (ø)
ixmp/core/platform.py	98.9% <98.9%> (ø)
ixmp/__init__.py	100.0% <100.0%> (ø)
ixmp/backend/__init__.py	100.0% <100.0%> (ø)
ixmp/backend/base.py	100.0% <100.0%> (+1.1%)	⬆️
ixmp/backend/jdbc.py	95.1% <100.0%> (+0.1%)	⬆️
... and 14 more