Behaves like the generic constraint but has optional spring on each axis.
TODO: Add option to set rest length.
Patch by Markus Kasten (markus111), thanks!
Constraints connect two rigid bodies.
Depending on which constraint is used different degrees of freedom
are limited, e.g. a hinge constraint only allows the objects to rotate
around a common axis.
Constraints are implemented as individual objects and bahave similar to
rigid bodies in terms of adding/removing/validating.
The position and orientation of the constraint object is the pivot point
of the constraint.
Constraints have their own group in the rigid body world.
To make connecting rigid bodies easier, there is a "Connect" operator that
creates an empty objects with a rigid body constraint connecting the selected
objects to active.
Currently the following constraints are implemented:
* Fixed
* Point
* Hinge
* Slider
* Piston
* Generic
Note: constraint limits aren't animatable yet).
