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.

 if WANT_TOOLS

 bin_PROGRAMS += \

 	tools/dimacs-solver \

 	tools/dimacs-to-lgf \

 	tools/lgf-gen

 dist_bin_SCRIPTS += tools/lemon-0.x-to-1.x.sh

 endif WANT_TOOLS

 tools_dimacs_solver_SOURCES = tools/dimacs-solver.cc

 tools_dimacs_to_lgf_SOURCES = tools/dimacs-to-lgf.cc

 tools_lgf_gen_SOURCES = tools/lgf-gen.cc