Some shifts deserve more compensation than other shifts. For example, if you
stay up all night or do a very difficult job then you should be rewarded.
So every shift has a weight. We multiply the number of hours in a shift by
the weight of that shift when calculating how many hours you're signed up for.
So if you sign up for a 4 hour shift with a weight of 1.5, then it counts as
if you had worked 6 hours.