Hmm. Good idea. But, I don't see how to do this. The base scalars, base_vectors _know_ the coordinate system they are in - but the CoordSys in itself doesn't know this.

We can have an iterator over `is_Vector == True` objects though. Or maybe we can just have a `@property` decorated methods that return the list to be iterated over (since we are iterating over very few items).