Implement NFCT/NFST

[tknopp]

There are the real versions of the NFFT called NFCT and NFST:

https://www-user.tu-chemnitz.de/~potts/nfft/fast_trig.php

In my vision we can nicely put them together into AbstractNFFT.jl and NFFT.jl.

To do this we can make the following four steps.

1. Introduce NFCT and NFST in AbstractNFFTs: This basically just means adding a line like this: https://github.com/JuliaMath/NFFT.jl/blob/master/AbstractNFFTs/src/interface.jl#L29
2. Implement the (slow) direct transforms: https://github.com/JuliaMath/NFFT.jl/blob/master/src/direct.jl#L2
3. Implement an NFFT3 wrapper (the NFFT3 has these transformations)
4. Implement a pure Julia version. We will likely move some files around when we are at this point.

FYI: @mischmi96

[mischmi96]

I have introduced the NFCT into AbstractNFFTs which required some adjustments in other areas to deal with the fact that we have real instead of complex numbers.

[mischmi96]

NDCT is also implemented now. I also fixed a bug where the small n was not reversed in the wrapper.

[mischmi96]

1 - 3 is done now. Wrapper works

I am also updating NFFT3.jl in order to directly implement the AbstractNFFTs Interface. However: Is there a way to reference a local dev version of package during development? The release of AbstractNFFTs does not yet have the NFCT implementation (of course, since it is only on the branch nfct). So I can't find a way that my NFFT3 dev version imports the AbstractNFFTs dev version on my computer instead of the release