Fix center of mass and inertia computations and add tests #127
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Previously, the combined center of mass of a body and its collider was computed as
com1 + com2, but this doesn't take into account different masses. The combined inertia was also computed asinertia1 + inertia2without taking into account mass or the center of mass, i.e. the offset.This PR fixes the center of mass issue by computing it correctly like
(com1 * mass1 + com2 * mass2) / (mass1 + mass2), and the inertia issue by first shifting the inertia values by the relative center of mass. Subtracting mass properties is handled similarly, but by simply using-instead of+in many places. These fixes will be very important especially when we implement child colliders.I also added tests to verify that the mass and center of mass are correct after adding or subtracting mass properties. I didn't add tests for inertia yet (other than checking that
self + other - other = self) as the calculations seem quite daunting to do by hand, so I'm not 100% sure if it's correct still. This can be verified later.