[ODE] const-correctness

[Matthew Koichi Grimes]

I noticed that large swaths of ode aren't const-correct. I'd be happy to 
submit patches, if the maintainers are interested.

PS: one project missing from the ode website's projects page is the 
DANCE engine, which has powered a few papers in SIGGRAPH and other 
conferences. It recently replaced its own dynamics code with ODE.
http://www.magix.ucla.edu/dance/

-- Matt