Use lru_cache for caching

[beatreichenbach]

First of all: I was blown away by your understanding of the math that goes into the FrFT, thank you so much for sharing it with the rest of us!

It could be nice to use lru_cache from the functools module to do the caching of the chirp function. I went ahead and rewrote some of your code with the use of the lru_cache decorator. https://gist.github.com/beatreichenbach/9f2eda2dd4cb16b75ebaececee89316f