SatelliteDataModule should check data paths in init

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.

Start at the SatelliteDataModule initializer and compare it with the train_dataloader, val_dataloader, and test_dataloader entry points mentioned in the issue. The work is done when each supplied data path is checked during instantiation, so a missing path fails before any dataloader method is called.

Written by the indexing model from the issue text.