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 

 coords	label	

 (-20,17)	15	

 (39,13) 	14	

 (39,-11)	13	

 (-12,7)	        12	

 (25,-15)        11	

 (-18,-14)	10	

 (45,3)          9	

 (28,13)         8	

 (25,-5)         7	

 (1,21)          6	

 (3,3)           5	

 (3,-9)          4	

 (-9,15)         3	

 (-13,-4)        2	

 (-27,5)         1	

 @edgeset 

 		label	

 1	15	22	

 8	14	20	

 11	13	18	

 1	12	8	

 4	11	14	

 1	10	1	

 14	9	21	

 13	9	19	

 8	9	17	

 7	9	16	

 11	9	15	

 5	8	12	

 6	8	11	

 5	7	13	

 3	6	4	

 12	5	9	

 2	5	6	

 3	5	5	

 2	4	10	

 10	4	7	

 15	3	23	

 1	3	3	

 1	2	2	

 @nodes

 source 1

 target 9

 @end