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.

 c LEMON max flow problem

 p max 7 9

 n 1 s

 n 7 t

 a 1 2 3

 a 1 3 2

 a 1 4 1

 a 2 5 3

 a 3 5 2

 a 3 7 5

 a 3 6 3

 a 4 6 1

 a 5 7 2

 a 6 7 4