Simplify and unify model configuration files

[vict0rsch]

New configs:

1. Provide a config argument string as {model}-{task}-{split}. Example: --config=sfarinet-is2re-all
2. The main (almost only) code change is in utils.py with the new load_config(config_str) function which processes the files this way:
   1. Load configs/tasks/{task}.yaml
      1. Read its default dict as initial config
      2. Update config with the task's {split} dict
   2. Load configs/{model}.yaml
      1. Update config from the model's default dict
      2. Update config from the model's {task}.default dict if it exists
      3. Update config from the model's {task}.{split} dict
   3. Tada 🎉
3. Mode is train by default (in {task}.yaml)
4. All previous configs are moved to a configs/legacy/ folder

[AlexDuvalinho]

Before merging, do you think we shall also change ase_utils.py, OCPCalculator ? It leverages (the "old") config_yml, which may cause troubles if the function is called, wouldn't it ?
It's not directly use for this purpose but in case we need it later, it may be good to adapt it now while it's fresh in our minds

Otherwise, it's great !

[vict0rsch]

Done :)