calc_waiting_list_pressure: Calculate Waiting List Pressure
Description
For a waiting list with target waiting time, the pressure on the
waiting list is twice the mean delay divided by the waiting list target.
The pressure of any given waiting list should be less than 1. If the
pressure is greater than 1 then the waiting list is most likely going to
miss its target. The waiting list pressure is calculated as follows:
pressure = 2 * mean_wait / target_wait.