Less map copying in NetworkSimplex (#234)

- The graph is copied in the constructor instead of the init() function.
It must not be modified after the class is constructed.
- The maps are copied once (instead of twice).
- Remove FlowMap, PotentialMap typedefs and flowMap(), pontentialMap()
setter functions.
- flowMap() and potentialMap() query functions copy the values into the
given map (reference) instead of returning a const reference to a
previously constructed map.

#cmakedefine HAVE_LONG_LONG 1

#cmakedefine HAVE_LP 1

#cmakedefine HAVE_MIP 1

#cmakedefine HAVE_GLPK 1

#cmakedefine HAVE_CPLEX 1

#cmakedefine HAVE_CLP 1

#cmakedefine HAVE_CBC 1