Reorganize physics classes

[neiser]

The number of physics classes is increasing, and some new user might not know where's a good place to look at. We could do:

• Reorganize folder structure, sort them more "logically" (but what is a good folder structure?)
• Give the Physics interface some methods ala void GetDescription(string& short, string& long) (default empty) enabling nice overview output for the main executable Ant
• Consistently name files after their contained physics classes, do not put more than one class into one file (for some that might create too many files)