Changed IBodyEvaluator -> IBodyProcessor, interface signature

Not all body evaluators will want to set a flag: some may want to alter
other body values, or apply smoothing to the joints.  This is a more
general implementation.
BodyProcessors will now be able to receive an enumerable of bodies

I have a use case where it's best to look at the entire list of bodies
rather than at them one by one.  Adding a new abstract base class to
the body processors, might get rid of the interface altogether in the
future.