A simple graph for test and debug

2 roads: an upper one of 2 cells and that can accept 2 unit of flow,
The lower road is only 1 cell-length but accept only 1 unit of flow.
Only a demand at time step 0, of 1.5 units of flow.

The system optimal solution should be: 2/3 going to the lower road and
1/3 on the upper road.