Separate interior state and boundary forcing to only predict state

[joeloskarsson]

Describe your changes

The goal of this PR is to establish a clear separation of the (predicted) state in the interior region and the boundary forcing coming from outside (and potentially overlapping with) the limited area. This PR performs these changes on the modeling side. That is from each batch is fetched from the dataset class and onward. Including how the tensors are propagated through the model, loss calculation, evaluation and plotting. This should be complemented with a separate PR for handling the data-loading side of things, where the boundary forcing could come from a separate dataset. That change should then build upon #66, once merged.

Note: In order to allow for working on this before the change has been done on the data loading side this currently includes changes in the MEPS npy dataset class that separates state and boundary already in there. This defines the interface between the dataset and model (currently missing #64 from #66, but that can easily be added later) and allows for working on these separately.

After this change:

• The model will only predict outputs within the limited area considered
• Plots will only include points within the limited area (this could be expanded in a future PR to plot also boundary fields, but would require a mapping between state and boundary forcing dimensions to plot together)
• Graphs will have to be created using a boundary mask as in Add a decoding mask option to only include subset of grid nodes in m2g weather-model-graphs#34 to make sure that the g2m-component only maps to the interior nodes.

Issue Link

No issue. This relates to the overall rework of Reading Training Data, but would be good to put as separate point on roadmap.

Type of change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📖 Documentation (Addition or improvements to documentation)

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