Function mass_ratio 

pub fn mass_ratio(delta_v: KmPerSec, ve: KmPerSec) -> f64

Tsiolkovsky mass ratio: m₀/m_f = exp(ΔV / vₑ).

Returns the ratio of initial mass to final (dry) mass required to achieve the given ΔV at the given exhaust velocity.

Panics if ve is not positive or delta_v is negative. Returns f64::INFINITY if ΔV/vₑ overflows exp().