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**Spring improvements**
This release makes the max force property on the two spring constraints work
as expected (previously it did not have any effect). It is also possible to
fully override the force/torque calculation of the springs with new callback.
Apart from this a collection of fixes in the underlying Chipmunk2D library
has been added. Finally, it's now possible to create static bodies with
constraints between them, and then change the bodies to dynamic to "start"
simulation of them.
Changes:
- Fix for 2 static bodies that are changed to dynamic and are attached to
constraints
- Updated the fork of Chipmunk2D used by Pymunk, fixing a number of issues,
including maxForce on Spring constraints.
- Added callback functions for DampedSpring and DampedRotarySpring to allow
customized force/torque calculations.