Add SpaceNet6 datamodule

[nilsleh]

This PR adds a DataModule for SpaceNet6. And makes the following changes:

• Introduce SpaceNetBaseDataModule
• Add dummy test data for SpaceNet1, it exists in the actual dataset, but not in the dummy data
• Add dummy data for SpaceNet6
• Resctructure tests/data/spacenet/ to generate test data for each version

Other Notes:

• The "test" data in SpaceNet comes without labels, so in the datamodules, I have configured them to be "predict" data.
• One could also restructure the data.py scripts, because there are shared functions that can be utilized across versions.
• it would be nice to have geospatial train/val/test splits generated from the train data, and not random splits

[adamjstewart]

Knowing full well how different each SpaceNet version is, I won't try to ask you to combine the data.py to avoid code duplication. Hopefully someday we can have data modules for all versions.