pub fn chi_minus(s: Float, m1: Float, m2: Float) -> Float
Computes $\chi_-(s, m_1, m_2) = 1 - \frac{(m_1 - m_2)^2}{s}$.
\chi_-(s, m_1, m_2) = 1 - \frac{(m_1 - m_2)^2}{s}