Fix the GEQ/LEQ handling in NetworkSimplex + improve doc (#291)

- Fix the optimality conditions for the GEQ/LEQ form.
- Fix the initialization of the algortihm. It ensures correct
solutions and it is much faster for the inequality forms.
- Fix the pivot rules to search all the arcs that have to be
allowed to get in the basis.
- Better block size for the Block Search pivot rule.
- Improve documentation of the problem and move it to a
separate page.

#cmakedefine LEMON_HAVE_LONG_LONG 1

#cmakedefine LEMON_HAVE_LP 1

#cmakedefine LEMON_HAVE_MIP 1

#cmakedefine LEMON_HAVE_GLPK 1

#cmakedefine LEMON_HAVE_CPLEX 1

#cmakedefine LEMON_HAVE_CLP 1

#cmakedefine LEMON_HAVE_CBC 1