(Effectively a re-write of the MassCalculation code)
- renamed some mass calculator accesor methods
- MassData, InertiaMatrix refactored into 'RigidBody' class
- refactors out cache code to separate wrapper class
- calculations now use the Transform class to translate masses, CM, and MOI
- new class: MassCalculation
- contains relevant context for a given calculation: tree-root, type, time, config
- contains most actual calculation code
- calculations are tracked with a context class: MassCalculation
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