cleaner switch to pyfftw (than the previous pull request)

After contacting https://github.com/hgomersall/pyFFTW, the 
pyfftw.interfaces.scipy_fftpack rfft and irfft were changed to have consistent input/output 
to the equivalent scipy.fftpack routines.

Previously pyfftw used numpy ordering of arrays, so one couldn't simply have a drop-in replacement. Using the newest git version of pyfftw, however, and one can simply drop in the much-faster pyfftw function calls.

This commit implements that.