`uds_diffusivity` settings API

[seanpearsonuk]

I have a couple of general questions about this section, i.e. as it already exists, and therefore not directed at @Gobot1234:

1. Terminology:
The quantities provided under uds_diffusivity have units of S/m (electrical conductivity) rather than any form of diffusivity (m²/s).
Is the term “diffusivity” here only a consequence of re-using the generic UDS diffusion operator internally? From a user perspective this is confusing, as the physical meaning is conductivity rather than diffusivity. The API user is not interested in Fluent's implementation details.

2. API structure:
The keys "uds-0" and "uds-1" appear to be created implicitly inside the uds_diffusivities container, with their semantics defined only by comments (electronic vs ionic conductivity).
This raises a few concerns:

• these objects are not discoverable in the public schema/docs;
• their meaning is position-based rather than name-based;
• nothing prevents them being swapped accidentally.

It would obviously make sense to expose a more explicit structure with named fields such as electronic_conductivity and ionic_conductivity, so that the API reflects the physical intent rather than relying on implicit indices?

Originally posted by @seanpearsonuk in #4887