Specialized stencil for kinetic energy

[simonbyrne]

To improve the performance of kinetic energy evaluation, we could define a specialized stencil operator for computing kinetic energy. i.e. it would take a Covariant12Vector-field on cell centers, and a Covariant3Vector-field on cell faces, and compute the kinetic energy reconstruction as per the atmos docs (https://clima.github.io/ClimaAtmos.jl/dev/equations/)

Would need to define stencil_interior method.

Note that no boundary handling should be necessary (as we set the vertical velocity at the boundaries appropriately)

Usage

K = Operators.KineticEnergyReconstruction()
k .= K.(uₕ,uᵥ)