Rename "lightweight" and "litemode" file read options to "settings-only"?

[raph-luc]

From the very general "lightweight" to the more specific "settings-only"

Would require Fluent server-side changes

See #2280 (comment)

[ansjsia]

I think we also need to make it very clear, especially in terminology, that there are three different ways to read a file:

• Normal: Load the mesh and settings.
• Settings-only: Do not load the mesh, but load settings.
• Lightweight (PyFluent only): Load settings only at first, then load the mesh in the background.

Presently, it seems like we have "settings-only" implemented but it is called "lightweight".