FFTW needed?

[michael-petersen]

A test compilation where FFTW was accidentally not included seemed to suggest that cmake still needed FFTW_INCLUDE_DIRS. To be investigated...

[The9Cat]

FFTW is used by TransformFFT for the DFT computations in pyEXP and by gensph. We could switch off DFT computations and not compile gensph if fftw is not found. But I'm currently thinking that it might be better to require FFTW.

This bug has languished long enough. The easiest thing to do is require FFTW and close it out. Thoughts on this?

[michael-petersen]

This is addressed by #90. We haven't had any complaints about requiring FFTW, so I think this is an easy fix.