Add Vector helper

[dopplershift]

After seeing some of the complexities in implementing storm motion and shear, it seems like we need a Vector container to make it simpler to group vector components.

• A simple wrapper around a Quantity
• Support unpacking like a tuple
• Support math operations
• Provide .magnitude to get vector magnitude
• Maybe have methods for dot and cross
• Methods to simplify cross product with unit vectors (think k x vec)
• Easy creation from 2 or more components (with unit checking)
• Easy access to individual components

[dopplershift]

Thought I might need this for gradient, but I see that it doesn't really make sense since the semantics for the order of components of gradient can't be reliably tied to x,y,z (yet at least), so I think it's better to just return them as a tuple. Punting this from the milestone...