Vector operations with quantities

[adrn]

I think we should add dispatches for some operators between vectors and quantities. For example:

vec = cx.CartesianPos3D.from_([1., 2., 3.], "kpc")
q = u.Quantity(1., "kpc")
vec / q  # CartesianPos3D with dimensionless unit 
vec * q  # CartesianPos3D with kpc**2 unit

Thoughts?

[nstarman]

Yes. These should become CartesianGeneric3D (should probably be renamed to just Cartesian3D)