Friction parameters are missing

[johnblommers]

This is an enhancement request to add frictional parameters in the equations. The user can control them at startup time in order to provide more realism to the simulation.

I understand this was not your initial intent but this addition would be useful to folks like me who are tying to understand chaotic systems.

[rheactive]

Added friction (a single parameter is enough, I just included dissipative force, proportional to speed). You can even change it during the simulation.