pub fn delta_n_to_intensity_mjy(x: f64, delta_n: f64, t_cmb: f64) -> f64
Convert distortion Δn(x) to specific intensity ΔI_ν in MJy/sr.
ΔI_ν = (2hν³/c²) Δn(x) = (2hν³/c²) Δn(x)
where ν = x k_B T_0 / h.