Add OmegaConf config loading for all the Anemois

[gmertes]

In anemoi-inference we now have some code that loads the user config yaml through OmegaConf, with support for command-line overrides and default values. There is some extra code on top to support overriding list items.

If other Anemois like datasets also want to start using OmegaConf we could move the config loading code from inference to utils.

@b8raoult was also thinking about making the distinction between configs and settings. What is currently called a config in utils are actually settings.