Replace constants.py with data_config.yaml

[sadamov]

Summary This PR replaces the constants.py file with a data_config.yaml file. Dataset related settings can be defined by the user in the new yaml file. Training specific settings were added as additional flags to the train_model.py routine. All respective calls to the old files were replaced.

Rationale

• Using a Yaml file for data config gives much more flexibility for various datasets used in the community. It also facilitates the future use of forcing and boundary datasets. In a follow-up PR the dataset paths will be defined in the yaml file, removing the dependency on a pre-structured /data folder.
• It is best practice to define user input in a yaml file, the usage of python scripts for that purpose is not common.
• The old constants.py actually combined both constants and variables, many "constants" should rather be flags to train_models.py
• The introduction of a new ConfigClass in utils.py allows for very specific queries of the yaml and calculations based thereon. This branch shows future possibilities of such a class https://github.com/joeloskarsson/neural-lam/tree/feature_dataset_yaml

Testing Both training and evaluation of the model were succesfully tested with the meps_example dataset.

Note @leifdenby Could you invite Thomas R. to this repo, in case he wanted to give his input on the yaml file? This PR should mostly serve as a basis for discussion. Maybe we should add more information to the yaml file as you outline in https://github.com/mllam/mllam-data-prep. I think we should always keep in mind how the repository will look like with realistic boundary conditions and zarr-archives as data-input.

This PR solves parts of https://github.com/joeloskarsson/neural-lam/issues/23

[leifdenby]

From a quick glance this looks simply amazing @sadamov! Thanks for doing this work. I will give a thorough review later today/tomorrow. Just tagging @SimonKamuk to have a read and give your thoughts too. I've added @ThomasRieutord to the organisation too. I'll also send Thomas an email so that he definitely sees the PR.

[sadamov]

I implemented most requested changes in the latest commit and requested one more review. From my side we are clear to merge. The latest changes were again tested for model training and evaluation.

[leifdenby]

Remember to update the changelog before you merge @sadamov! I think I would call this a new (very useful!) feature. A few things also change here (where constants are stored and thus how they are accessed in the code)

[leifdenby]

Hurraay! :partying_face:

[joeloskarsson]

Wonderful job with this @sadamov!