In empty loading unit repositioning, there is a set of empty and reusable loading units that should be shipped back from the location
they are emptied to the locations they are needed. These loading units do not directly incorporate in the profit of transportation,
but guarantee requested service level. In empty repositioning problem, future customer demand is unknown and the objective
function is to minimize empty transportation plus storage costs, stockout costs, and in some cases, substitution costs. These problems
are usually modeled as NFP problems with continuous variables to represent the movement of the loading units.