Function rl_lowpass_magnitude 

pub fn rl_lowpass_magnitude(l_henrys: f64, r_ohms: f64, freq_hz: f64) -> f64

First-order RL lowpass frequency response magnitude at given frequency.

For RL lowpass: L in series with input, R to ground (output across R) |H(f)| = 1 / sqrt(1 + (f/fc)^2) where fc = R / (2piL)