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.

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