Adding anaerobic digester cost

[agarciadiego]

Fixes/Resolves:

Addresses #1360 for Anaerobic Digester

Summary/Motivation:

Required for WaterTap 1.0

Changes proposed in this PR:

• Adds documentation for anaerobic digester

Legal Acknowledgement

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1. I agree my contributions are submitted under the license terms described in the LICENSE.txt file at the top level of this directory.
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[MarcusHolly]

A few typos

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.94%. Comparing base (41b6ae3) to head (78bdb13).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1433   +/-   ##
=======================================
  Coverage   93.94%   93.94%           
=======================================
  Files         311      311           
  Lines       33151    33151           
=======================================
  Hits        31143    31143           
  Misses       2008     2008           

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