Add Fourier interferometers explicitly to Perceval

[anthonyrtw]

Fourier interferometers are possibly the most commonly used optical component/circuit that is not directly available in Perceval currently.

Here, I've defined a Fourier interferometer class UFT(Unitary) that can be immediately defined:

from perceval import *
fourier_circuit = Circuit(m) // UFT(m)

This saves the user from calculating the Matrix form of Fourier interferometers and then explicitly defining the matrix as a Unitary.

I have also made it available from root.