Improve network simplex algorithm

- Remove "Limited Search" and "Combined" pivot rules.
- Add a new pivot rule "Altering Candidate List".
- Make the edge selection faster in every pivot rule.
- Set the default rule to "Block Search".
- Doc improvements.

The algorithm became about 15-35 percent faster on various input files.
"Block Search" pivot rule proved to be by far the fastest on all inputs.

 Document: lemon-refman

 Title: LEMON Reference Manual

 Author: LEMON Development Team

 Abstract: Documentation of LEMON.

 Section: unknown

 Format: HTML

 Index: /usr/share/doc/lemon/html/index.html

 Files: /usr/share/doc/lemon/html/*.html