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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2 	6 	8 	  8

5 	3 	9 	  20

3 	6 	10 	  25

4 	7 	11 	  15

5 	7 	12 	  15

6 	7 	13 	  18

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