replace filon FT with numpy.fft doublesided

Given how much trouble Filon seems to give us, and that Fredrik have shown that it is pretty much identical to double sided np.fft maybe we should replace Filon with numpy.fft?

@EsmeeBerger @erikfransson @freeriks @erhart Any objections?

Then we can also just clearly state that we do FourierTransforms using numpy (as is standard) and not get into trouble.

Also numpy fft should be a lot faster than Filon?