Manipulating `AxisTensor`s

I'm unsure of the support or best way to manipulate `AxisTensor` objects like `U` `V` and `WVector`s, such as taking a norm or inner product. One way is of course to manipulate the array with parent, but then I'd have to repackage the result in a Field.

Motivation:
In #325 we add the ability to specify boundary conditions as fields, so that we can have a location-varying flux applied at the boundaries. I want to easily be able to compute the wind stress at every point on a boundary field and have this represented as a scalar valued Field.

A current workaround can be seen in [ClimaAtmos](https://github.com/CliMA/ClimaAtmos.jl/blob/06a4b7e7143f45fa2c2e724940e8eb829c5ac3eb/src/Models/SingleColumnModels/equations_vertical_diffusion.jl#L59-L60) or [ClimaCoupler](https://github.com/CliMA/ClimaCoupler.jl/blob/4f6534f511055ea1e47a955c2ea4c131df2998ec/experiments/ClimaCore/sea_breeze/coupledbc.jl#L18-L19) in which single elements of vectors are used to calculate this stress which is then uniform on the boundary.

@akshaysridhar please chime in, I think we have different understandings of the issue here.