Further Prune PostProcessing Code, Specifically plot_posterior And spread_draws

[AFg6K7h4fhy2]

This PR:

In an effort to leave post-processing to the user, DB + others have determined that plot_posterior and spread_draws, stemming from the mcmcutils.py and being used partially in metaclass.py can be pruned.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.53%. Comparing base (ade84ce) to head (51c4c8b).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #431      +/-   ##
==========================================
+ Coverage   94.03%   96.53%   +2.49%     
==========================================
  Files          42       41       -1     
  Lines         989      952      -37     
==========================================
- Hits          930      919      -11     
+ Misses         59       33      -26     

Flag	Coverage Δ
unittests	96.53% <ø> (+2.49%)	⬆️

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