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.

 LEMON as a whole is copyrighted by Egerváry Jenő Kombinatorikus

 Optimalizálási Kutatócsoport (Egervary Combinatorial Optimization

 Research Group, EGRES).

 You are free to use, modify and distribute LEMON under the terms

 described in the LICENSE file.

 This project is supported by

 * Egyetemközi Távközlési és Informatikai Központ (Inter-University

   Centre for Telecommunications and Informatics, Hungary)

 * Traffic Analysis and Network Performance Laboratory,

   Ericsson Research, Hungary

 * Department of Operations Research, Eötvös Loránd University,

   Budapest