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**Spring improvements**
This release makes the max force property on the two spring constraints work
as expected (previosuly it did not have any effect). It is also possible to
fully override the force/torque calculation of the springs with new callback.
Appart from this a collection of fixes in the underlying Chipmunk2D library
has been added. Finally its 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 constaints.
- Added callback functions for DampedSpring and DampedRotarySpring to allow
customized force/torque calculations.