I think `partial_velocity` can go in the vector calculus package. Partial velocities may be used in places other than Kane's method. A google search for "parital velocity" turns up various hits. But I don't really know if it is the same. It can also be added to the kane module in mechanics, since that is the only place we use it. Maybe that makes more sense.

The motion parameter function seems fine to be in the vector calculus package too.