Improve FFT performance

[ckormanyos]

The FFT implementation is very rudimentary. Use more refined FFT or DHT to improve performance.

[ckormanyos]

Also note that the precision of the current rudimentary FFT implementation does not seem to be as good as hoped (or maybe could be). Can the precision be improved --- even for the rudimentary FFT implementation?

[ckormanyos]

A simple improvement resulting in reduced runtime would be to support FFTs having lengths of 2^k times 1 or 3, instead of just 2^k times 1.

[ckormanyos]

See also #126