pub fn rho(s: f64, m1: f64, m2: f64) -> Complex64
Computes the phase-space factor $\rho(s, m_1, m_2) = \sqrt(\chi_+(s, m_1, m_2)\chi_-(s, m_1, m_2))$
\rho(s, m_1, m_2) = \sqrt(\chi_+(s, m_1, m_2)\chi_-(s, m_1, m_2))