Making SplineInterpolator generic

[Lehonti]

No description provided.

[cameronwhite]

I don't have any real objection to the change, but I'm curious if you have a use case that benefits from using float instead?

[Lehonti]

This is a mathematical algorithm, so I wanted to make the code more 'matematically pure'. Having the numeric type abstracted out like this would make it read a bit more like a piece of mathematical reasoning (because it makes it clearer which part of the code corresponds to which part of the reasoning).

I see permissively-licensed projects like these as gifts to humankind, and having its distinct parts be clear and exemplary or at least decent reference implementations of something would give that idea some justice.

Those are my thoughts, but let me know yours :)