Support pyFFTW along with PyFFTW3

[liujimj]

I couldn't get PyFFTW3 installed on my machine, but pyFFTW is an alternative wrapper for FFTW that appears better maintained.

You can see a speed comparison of the two wrappers here, they're more or less equivalent:
mperrin/webbpsf#10

I'll submit a pull request in <1 week to get it working.

[carlthome]

@liujimj, did you ever get around to doing this?

[liujimj]

@carlthome See #8

@grrrr Could you please look again at my responses on the PR?