update fpca references

[aleexarias]

Resolves #634

The reference in _fpca is moved into the class docstring instead of the private function it was on, using footcite/footbibliography

• [X] I have performed a self-review of my code
• [X] The code conforms to the style used in this package
• [X] The code is fully documented and typed (type-checked with Mypy)
• [X] I have added thorough tests for the new/changed functionality

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 86.83%. Comparing base (2f48baa) to head (68cb73e). Report is 4 commits behind head on develop.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #636 +/- ## ======================================== Coverage 86.83% 86.83% ======================================== Files 157 157 Lines 13522 13522 ======================================== Hits 11742 11742 Misses 1780 1780 ```

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