Probability to the left or right of a number, inclusive or not.
prob_left() is a more general cdf defined using either < or <=, and
prob_right() is a more general survival function defined using either
> or >=.
Examples
d <- dst_pois(5)
prob_left(d, of = 3, inclusive = TRUE)
#> [1] 0.2650259
prob_left(d, of = 3, inclusive = FALSE)
#> [1] 0.124652
prob_right(d, of = 0:3, inclusive = TRUE)
#> [1] 1.0000000 0.9932621 0.9595723 0.8753480
