Note concerning the coding of the form factors FF
In the coding of the different form factors, we have taken the
following convention:
3D : V(q) = e^2 / (EPS_0 * q^2) * FF
2D : V(q) = e^2 / (2*EPS_0 * q) * FF
1D : V(q) = e^2 / EPS_0 * FF