Remove Smart Pointer for Network object.

[var-nan]

Network object is created typically and passed to the DDSolver object in the main function. Since the network object is destroyed automatically after the main() scope, we can just pass the normal pointer (pointer to const network object) to the DDSolver.

Network pointer is copied multiple times for every cut generation, with an overhead of updating the reference counts. Since the Network object remains constant after its creation, we can use pass const pointer to const Network object to the solver.