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 

 label

 6

 5

 4

 3

 2

 1

 0

 @edgeset 

                 label      length

 5       6       9       4

 4       6       8       2

 3       5       7       1

 2       5       6       3

 2       6       5       5

 2       4       4       2

 1       4       3       3

 0       3       2       1

 0       2       1       2

 0       1       0       3

 @nodes 

 source 0

 target 6

 @edges 

 @attributes 

 @end