removed overwritten methods
introduce checks whether a Body has SpringPhysics and PressurePhysics
added getter for SpringPhysics and PressurePhysics
removed direct access to gasPressure in Body
removed direct access to volume in Body
moved accumulateInternalForces implementation to Body
removed direct access to addInternalSpring in Body
removed clearAllSprings from Body
moved spring building from BodyBuilder to SpringPhysics
removed constant modifier in Body
removed type attribute from Body
removed guard for non-visible Bodies in SpringPhysics
introduced separate file for Body.fromJson to avoid circular
dependencies
removed unused inclusions in Body.fromJson