AbstractFFTs instead of FFTW – githubmate

ImageCore softly depends on FFTW with @require just to provide the suggestive error messages (cf. #106).
However, for several years now, its APIs have been defined in AbstractFFTs. Therefore, I see no reason to limit the target to FFTW.

I think a hard dependence on AbstractFFTs would eliminate the need to use Requires, but given that ImageCore doesn’t actually provide FFT functionality, it’s also reasonable not to have a hard dependence.

I don’t know if it is necessary to use “genuine” FFTW for testing. Personally, it seems to be sufficient that an image package which actually uses FFTW tests the transformation.