Correction of mass conservation deerivative terms
Small refactoring
Simplifying the computation of dJ/dU
BUGFIX: The vehicles at the destinations were included in the cost function
BUGFIX: The supply of a sink is density/delta_t instead of just density
Feature: Now we also compute the evolution at time T-1 and drp the one at T

We want to have the correct values for the demand/supply/in and out-flows
even for the last state for time step T-1