getFftData only works for 2^k or 2^extended_k right now

[jonathanpwang]

If you want to use #9 (fft mem optimization) with the recursive FFT, you need to do FFT for lengths between 2^k and 2^extended_k. Right now it will give you erroneous FFT data in between.

I fixed this in my dev branch by just making domain.fft_data into a HashMap since usually k..=extended_k is not that big.