Make output types of Vectors, Dyadics consistent.

This continues the change started by this commit:
375e12d139b8305c36bac5b8ca7637e0521b4ebc.
Allow pos, vel, acc to be set to 0 in physics/mechanics.

Allow users to set pos, vel, acc, ang vel, ang acc to zero with 0 in
addition to the zero vector.