Eval Funcs docs

[gawng]

Purpose

This PR adds a documentation page of all the possible eval funcs (cost functions) straight out of pyADflowADFLOW

Expected time until merged

I still need to spend some time digging through the code to give proper definitions but I'm moving this to draft PR so it's more in my face that this is being worked on.

Type of change

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (non-backwards-compatible fix or feature)
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Documentation update
• Maintenance update
• Other (please describe)

Testing

Checklist

• I have run flake8 and black to make sure the Python code adheres to PEP-8 and is consistently formatted
• I have formatted the Fortran code with fprettify or C/C++ code with clang-format as applicable
• I have run unit and regression tests which pass locally with my changes
• I have added new tests that prove my fix is effective or that my feature works
• I have added necessary documentation