Add .util.slum and CLI

[khaeru]

This PR adds a mix-models sbatch CLI command that simplifies usage with SLURM, for instance on UnICC.

Closes #284.

How to review

• Read the added documentation via the preview build.
• Check out the branch on UnICC
• Run a command like mix-models sbatch -- config show.

More TBA

PR checklist

• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation.
• Update doc/whatsnew.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.0%. Comparing base (b920f59) to head (5385d66).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #291   +/-   ##
=====================================
  Coverage   77.0%   77.0%           
=====================================
  Files        213     213           
  Lines      16512   16512           
=====================================
  Hits       12724   12724           
  Misses      3788    3788           

Files with missing lines	Coverage Δ
message_ix_models/cli.py	93.4% <ø> (ø)