Start using fft.rfft instead of fft.fft

Using `fft.fft` was very useful in early stages to properly understand the math, but it is quite inefficient (both in memory and time).

We should figure out how to use `fft.rfft` instead, write down the changes in Overleaf first, implement it to code, and check that we do get exactly the same result (to machine precision, I guess).