Change of angular velocity integration method.

[GrzegorzSzczodrzec]

Angular velocity should be computed in the rotated coordinate system to get the proper value of the angular momentum derivative: https://en.wikipedia.org/wiki/Euler%27s_equation_of_motion

The provided code change switches angular velocity integration to the local (rotating) coordinate system.

A unit test is added. In the absence of external torque (caused by gravity or contact), the angular momentum vector should remain constant. The included unit test verifies this property.