Solves Rankine-Hugoniot relations at a shock with theta_Bn. The derivation follows "Effect of Oblique Interplanetary Magnetic Field on Shape and Behavior of the Magnetosphere", by G. Walters, JGR 1964, https://doi.org/10.1029/JZ069i009p01769
Pages 1-4 explain the derivation from equations A''3, A''4 and A''6. The Python code solves for the density ratio (eta) and plots it as a function of thetaBn, the angle between the IMF (Interplanetary Magnetic Field) and the shock normal.