Added `to_matrix` method to Vector and Dyadic in mechanics.

When jumping back and forth from symbolics to numerics when using the mechanics
module it would be nice to be able to output the matrix forms of vectors and
dyadics with respect to different reference frames. I've added two new methods
that provide this functionality. It is particularly useful for returning
inertia tensors in reference frames from dyadics as the tensor form is the more
widely used.