Implement sqrt(N) Fourier and related transforms on a grid

[kevinsung]

From Section V of https://arxiv.org/pdf/1711.05395.pdf.

I have some ugly code from before the days of Cirq to implement the transformation Gamma from Eq. (38). You can find it here: https://github.com/kevinsung/fermions/blob/master/gamma/gamma.ipynb. Most of the implementation is contained in the second and third cells. Perhaps this will be of some help.

@mrwojtek