[ODE] Reference counters

[Lukasz Stafiniak]

Hi!

I am writing a memory-safe port of ODE for the OCaml language. For
this, I needed reference counters for dxBody, dxJoint, dxGeom: this
adds one pointer to these structures (initializing it to 0), calling a
macro when destroying them, and destroying them conditionally in
cleanup mode (this could be a third cleanup mode, for this is a
convenience and not a necessity). What do you think about it? Could it
be included into the main ODE?

Best Regards,
Lukasz Stafiniak