Add mermaidjs based code diagram

[leifdenby]

To aid discussion around refactoring the code and allow for easy modification of the code diagram in future I have created a mermaidjs based diagram in the README in place of the graphic that was there previously. Please have a look @joeloskarsson if you think this covers what the old diagram did.

Here's a direct link in the README: https://github.com/leifdenby/neural-lam/tree/feat/readme-code-diagram?tab=readme-ov-file#pre-processing, and a screenshot from vscode:

[leifdenby]

@joeloskarsson I updated the diagram because I realised that I had slightly misunderstood the shapes of what is in the .npy-files. I made the following notebook which plots each of the inputs you use, just for myself to learn what the shapes of what everything is: https://github.com/leifdenby/neural-lam/blob/examining-inputs/notebooks/examining_inputs.ipynb