Add original pandana citation for users

[jGaboardi]

• This PR adds the original pandana citation for ease of user access

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (ad52f75) to head (a01e4d6).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main     #44   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
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  Files          5       5           
  Lines        297     297           
=====================================
  Misses       297     297           

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