Function larmor_precession 

pub fn larmor_precession(state: &SpinState, b_field: Vec3, dt: f64) -> SpinState

Larmor precession: time evolution in a magnetic field. H = -gamma * B.sigma, evolve by exp(-iHt/hbar). For simplicity, gamma*hbar/2 = 1, so omega = |B|.