Cartesian xyz to lon, lat and zlevel

[bjlittle]

✨ Feature Request

Motivation

There are users with a workflow that generates point clouds, which they then serialize to .vtk.

They then wish to load the .vtk and convert the xyz points on the mesh back to lons, lats and zlevels.

At present it is possible to achieve this for lons and lats, but not for zlevels.

We require an enhancement to the following functions:

• iris.common.to_lonlat
• iris.common.to_lonlats
• iris.common.from_cartesian

to extend their behaviour to also return the zlevel, if requested i.e., this is an opt-in behaviour.

The use case for this enhancement is driven by the ORCA2 Point Cloud example and this GH Discussion request from @berndbecker.