Add Z axis support for spatial averaging

[tomvothecoder]

Discussed in https://github.com/xCDAT/xcdat/discussions/591

^{Originally posted by tomvothecoder January 29, 2024}

Is your feature request related to a problem?

I'm running into a case where I need to average over the Z axis. The CDAT code being used is cdutil.averager(tvar, axis="z"), which I need to replace.

xCDAT's spatial averager, which is based on cdutil.averager(), only supports rectilinear grids ("X" and "Y"). However, we do mention that the get_weights() method can be extended to support other axes: https://github.com/xCDAT/xcdat/blob/fbf1db68fd6328d5a534e0c1e57e22e6fdd70c76/xcdat/spatial.py#L246-L252

Are there are any possible answers you came across?

No response

Describe alternatives you've considered

No response

Additional context

For the short-term, I might be able to use xarray.DataArray.weighted directly. However, I still need to generate the weights xr.DataArray beforehand.

datarray.weighted(weights).mean("height")

[pochedls]

@tomvothecoder - FYI - I thought this might only be a couple lines and took a look. It ended up being a little more than I thought, but I committed a working prototype because I thought it might be helpful (needs validation, review to make sure the docs are updated correctly, and unit tests). It could be possible to combine some of the _get_XYZ_weights functionality, but maybe it is fine as-is. We can delete the branch if you have something in progress already. Or I can open a PR.

[tomvothecoder]

@pochedls Thanks for taking a stab at this so quickly! You can open a PR and tag me for review.

As a temporary workaround in e3sm_diags, I created a _get_z_weights() function that is based on xCDAT logic. I am planning on replacing it whenever xCDAT supports generating weights for the Z axis, which now seems sooner rather than later.