FPCA Basis

[luisheb]

Describe the proposed changes

FPCA basis added to facilitate its creation without having to use a custom basis.

Checklist before requesting a review

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

[luisheb]

I haven’t added any tests for this new functionality, as it didn’t seem strictly necessary for this implementation. That said, if you think tests should be included, I’m happy to add them.