Function spd_to_xyz 

pub fn spd_to_xyz(spd: &Spd) -> Xyz

Project an SPD through the CIE 1931 2-degree CMFs to obtain XYZ tristimulus values.

This is the linear projection: X = Σ S(λ)·x̄(λ)·Δλ, etc. The result is normalised so that an equal-energy SPD yields the D65 white point.