Use XTI implementation instead of ATI in NetworkSimplex (#234)

XTI (eXtended Threaded Index) is an imporved version of the widely
known ATI (Augmented Threaded Index) method for storing and updating
the spanning tree structure in Network Simplex algorithms.

In the ATI data structure three indices are stored for each node:
predecessor, thread and depth. In the XTI data structure depth is
replaced by the number of successors and the last successor
(according to the thread index).

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