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.
Return a Vector object in Vector method diff.

This continues the transition of this commit:
375e12d139b8305c36bac5b8ca7637e0521b4ebc.