Implement NFCT/NFST

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