[ODE] fixed joints

[Fig TaTam]

I get the idea that fixed joints are unwanted in ODE. But why? They may be 
uninteresting for the simulation, but they can be useful in practical 
applications.

For example, lets say I have a car, and I want to represent car engine as a 
separate object. Why would I want to do that? because I want to be able to 
customize a car with different engines. Engines could have different mass 
properties. Now what if I want to add some external attachments? customize 
car exterior. Then it's important to have Geom class attached properly for 
collision detection. There needs to be a simple way to do this using ODE.

Right now, I can't use fixed joints because there's no rotation offset, just 
position.

Maybe if the "disable body" function worked, I could make a hack to make 
body disappear when it's attached, but instead, I have to manually set 
position and rotation of that body every update cycle. And I have to pass 
any forces it receives to the "other" body, to which attachment is made.

I just think this should be handled by ODE.

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