CustomLengthConstraint migration

This class is no longer exposed in Python. In C++ it remains only as a source-compatibility view of a two-site tendon, returned by World::addCustomWire. For new code, use a two-site tendon and call Tendon::setTension (positive tension pulls) or Tendon::setActuationForce (positive force lengthens the tendon). See Constraints for the migration table and Tendon code examples for complete, compiled examples.