real get_eta(const real *q, const real J[3], const bool const_resistivity, const real constant_resistivity_eta, const std::string resistivity_type, const real max_eta, const real min_eta, const real n_vac, const real eta_vac, const real gas_gamma, const real ompt, const real nupt, const real lnlam, const real Ai, const real Zi, const real Ae, const real rho_min, const real press_min)