Add More Costing Documentation

[MarcusHolly]

Fixes/Resolves:

Issue #1360

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.92%. Comparing base (abe21bc) to head (f5ef4cf).
Report is 60 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1395   +/-   ##
=======================================
  Coverage   93.92%   93.92%           
=======================================
  Files         335      335           
  Lines       35620    35620           
=======================================
  Hits        33456    33456           
  Misses       2164     2164           

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[MarcusHolly]

Note to reviewers: If you happen to know where the unit model costing data came from for ERD, nanofiltration, pressure exchanger, pump, or RO, let me know and I can add the reference(s) to the documentation. Otherwise, we can just merge these in without references.

[savannahsakhai]

LGTM

[agarciadiego]

LGTM