Move the code deserializing contact manifolds from MultiBodyWorldImporter to WorldImporter

[nbelakovski]

That code (the templated function syncContactManifolds) has no dependencies on anything in MultiBodyWorld, or even in DiscreteDynamicsWorld or DynamicsWorld. Moving it into the WorldImporter would allow more people to use it without having to use a MultiBodyWorld, and would be the right place for it in general given that it has no dependencies that prevent it from being there.