Draft: Define new config object to be used throughout the repository

This MR defines a new configuration object to be used throughout the repository.

The configuration object pipeline_config (or CONFIG, whatever you wanna call it) is an instance of PipelineConfig.

Having an class object instead of a mere dictionary allow to define methods and attributes directly attached to that object, avoiding functions defined throughput the repository.

For instance, to load the model of the gnn step through the pipeline_config, you can write

gnn_model = pipeline_config["gnn"].load_trained_model()

Instead of doing

from pipeline import load_trained_model
gnn_moodel = loaded_trained_model(path_or_config=pipeline_config, step="gnn")

(so you'd have to know the function was defined in pipeline/__init__.py...).

Still WIP