ef_fun
returns amount of vehicles at each age
numeric vector.
https://en.wikipedia.org/wiki/Logistic_function
if (FALSE) { # \dontrun{
CO <- ef_cetesb(p = "CO", veh = "PC_G")
ef_logit <- ef_fun(ef = CO, x0 = 27, k = 0.4, L = max(CO))
df <- data.frame(CO, ef_logit)
colplot(df)
} # }