Calculate the percentile waiting at a given week

Usage

calc_percentile_at_week(
  wl_shape,
  week,
  wlsize_col = "wlsize",
  time_col = "months_waited_id"
)

wl_shape: a tibble with columns for the number of months waited (where 0 is 0-1 month) and volume waiting
week: numeric; the week to calculate the percentile for
wlsize_col: character length 1; the name of the column containing the volume waiting
time_col: character length 1; the name of the column containing the number of months waiting