Choose a naming for "pre_processors" vs "preprocessors" (and "post_processors" vs "postprocessors")

[floriankrb]

To make the code consistent, I think we should choose one convention:

Either use pre_processor and PreProcessor and "pre-processor"
Or use preprocessor and Preprocessor and "preprocessor"

We currently have a mix of both.

https://github.com/search?q=repo%3Aecmwf%2Fanemoi-models%20preprocessor&type=code
https://github.com/search?q=repo%3Aecmwf%2Fanemoi-models%20pre_processor&type=code

Same thing for post-processors.

I have no preference but mixing names is not ideal. Any opinion?

[floriankrb]

Thumbs up here for "pre_processor"

[floriankrb]

Thumbs up here for "preprocessor"