Major improvements in NetworkSimplex.

Main changes:
- Use -potenital[] instead of potential[] to conform to the usual
terminology.
- Use function parameter instead of #define commands to select pivot rule.
- Use much faster implementation for the candidate list pivot rule.
It is about 5-20 times faster now.
- Add a new pivot rule called "Limited Search" that is a modified
version of "Block Search". It is about 25 percent faster on rather
sparse graphs.
- By default "Limited Search" is used for sparse graphs and
"Block Search" is used otherwise. This combined method is the most
efficient on every input class.
- Change the name of private members to start with "_".
- Change the name of function parameters not to start with "_".
- Remove unnecessary documentation for private members.
- Many doc improvements.

@nodeset 

coordinates_x	coordinates_y	f	label	

-198	-73	2	7	

120	30	4	6	

-17	222	1	5	

328	147	1	4	

247	-179	0	3	

-28	-93	1	2	

-131	83	3	1	

@edgeset 

		label	

7	6	3	

1	6	2	

2	6	1	

4	5	10	

6	5	6	

3	4	9	

6	4	7	

2	3	12	

6	3	8	

7	2	4	

5	1	11	

7	1	5	

@nodes 

@edges 

@attributes 

@end