Allow users to build Amplitudes in Python

[denehoffman]

This is currently sort of possible to implement, but it won't be very useful due to the GIL. I did this in rustitude and the same thing can work here, but I don't really want to implement it till freethreading Python becomes available in PyO3 (it currently is, but rust-numpy doesn't yet support it). When this is solved, it shouldn't be terribly hard to implement, but I'm not sure how easy it will be to use.