SatelliteDataModule should check data paths in init

[dfulu]

The SatelliteDataModule object should check the supplied data paths exist at instantiation. Otherwise we may only get an error after calling its .[train/val/test]_dataloader() method.

[Alkholy53]

I've created a pull request to address this issue. The changes include:

1. Added a new _validate_data_paths() method to check if all provided data paths exist
2. Added validation call in __init__ after setting the zarr_path attribute
3. Added proper error handling with descriptive messages

The validation will now happen at instantiation time, and will raise a FileNotFoundError with a descriptive message if any of the data paths don't exist. This helps catch data path issues early, before any dataloader methods are called.

You can find the pull request here: #92

The changes ensure that invalid data paths are caught early during initialization rather than when the dataloader methods are called, making it easier to debug configuration issues.
@dfulu can you review it