slowdown_func — slowdown

function to evaluate a slowdown momentum distribution, https://arxiv.org/abs/1808.01934v2, eq. 2.9

slowdown_func(p_perp, p_par, n, K, p_c, p_b, b)

Arguments

p_perp	`numeric` value of perpendicular momentum component
p_par	`numeric` value of parallel momentum component
n	`numeric` particle density.
K	`numeric` integration constant
p_c	`numeric` critical momentum
p_b	`numeric` birth momentum
b	`numeric` transport parameter

numeric value of momentum distribution at (p_perp, p_par)