Broadcasting of vectors and fields

Currently it's somewhat frustrating when trying to broadcast vector quantities over a field, as the axis don't match. Ideally we would have some way to resolve this conflict (i.e. when both appear, only broadcast over a field?). If not, we could make `AxisTensor`s act like scalars (i.e. would no longer be `AbstractArray`s).

Also, make `Axis` objects act like scalars so you don't need a `Ref` when broadcasting `transform`.