Fix docstring for outer function.
Consistency w/ 0s in mechanics

Previously, there were inconsistencies in mechanics.essential, with some
methods using S(0) to form empty Vectors and others using an empty Vector
(Vector([])). This has now been corrected to use empty vectors only. The same
is true for the Dyadic class.
Add test for output types of Vector, Dyadic products.

Test the output types for dot, cross, and outer products of Vectors and
Dyadics.
Change zero equality testing in Vector, Dyadic.

Move zero equality testing into __eq__ method for Vector, Dyadic. Remove
zero case in _check_vector, _check_dyadic.
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.
Test dot, cross, outer functions match operator methods.
Add case to create zero Vector, Dyadic with constructors.
Fix zero vector in ReferenceFrame.dt().
Replace simplify() in physics/mechanics test.