Towards #779: [Sim, make $] Easier benchmarking, via new 'multisim' CLI

[trentmc]

Towards #779: everything but multiprocess

• New MultisimSS module, plus test
• New MultisimEngine module, plus test
• PPSS can now optionally be instantiated directly with a constructor dict d
• New PointMeta module, to define a space of >1 Point (also new).
• New utils: obj_in_objlist(), keyval(), flat_to_nested_args(), ..
• Support different multifeeds. Commit 40a79c4

Also fixes:

• #797 "[CLI, UX] CLI help is too long for new users". Commit 5a5f288
• #798 "[Logging, UX] I can't tell which logger is outputting which text". Commit 23f295c
• #789 "[Warning, Aimodel] test_sim_engine has UndefinedMetricWarning in classification". Commit 3c5a7dc
• #677 "[Warning, CI] In test_mathutil, get 'FutureWarning: Downcasting object dtype arrays'". Commit c8a9c9e