Support multiple attachment points

[Stefano Balzarotti]

Can you switch the current physic engine model to use graphs instead of trees, with the purpose to support multiple attachment points?
Actually with current tree model it's impossible to create a strong rigid vessel with a complex multiple parts structure.
It's possible to reinforce the structure with "Struct Connector", but this is a trick, and a connector is weak to strong forces, it's really bad to see wobbling the parts simply because you can't connect them together.