Generalization of OrthogonalSphericalShellGrid to be able to construct _any_ grid with coords-metrics that vary in both i, j

[navidcy]

At the moment the OrthogonalSphericalShellGrid constructor is only specialized to conformal cubed sphere mapping. But OrthogonalSphericalShellGrid can be more general to that. We should add a conformal_cubed_sphere_panel constructor for the cubed sphere panel. We should also gather all conformal-cubed-sphere-related properties like ξₗ, ξᵣ, ηₗ, ηᵣ into a property e.g. conformal_cubed_sphere.