Add BigEarthNet dataset

[isaaccorley]

• Adds torchgeo.datasets.BigEarthNet
• Recently BigEarthNet was updated with coregistered Sentinel-1 patches to the original Sentinel-2 patches referred to as BigEarthNet-MM. This dataset handles loading either Sentinel-1 bands, Sentinel-2 bands, or both.

Notes:

• I'm not using Radiant MLHub's formatted version of the dataset because they don't include the Sentinel-1 patches which were included to create the BigEarthNet-MM dataset. See bigearth.net for more info.
• BigEarthNet._load_paths is really not elegant but it's due to the mapping between Sentinel-1 -> Sentinel-2 patches being in the individual json file for each Sentinel-1 patch. I'm computing the mapping on indexing and not on instantiation of the dataset because opening 590k json files is too time consuming for instantiating a dataset.
• I made non georeferenced dummy images for tests so rasterio throws a warning during tests related to this

Closes #63

[adamjstewart]

I see the following warnings in our tests:

tests/datasets/test_bigearthnet.py::TestBigEarthNet::test_getitem[all]
tests/datasets/test_bigearthnet.py::TestBigEarthNet::test_getitem[s1]
tests/datasets/test_bigearthnet.py::TestBigEarthNet::test_getitem[s2]
  /opt/hostedtoolcache/Python/3.6.15/x64/lib/python3.6/site-packages/rasterio/__init__.py:220: NotGeoreferencedWarning: Dataset has no geotransform, gcps, or rpcs. The identity matrix be returned.
    s = DatasetReader(path, driver=driver, sharing=sharing, **kwargs)

Can you look into this? It seems like the test files are not actually GeoTIFFs but we're still using rasterio to load them. Are the actual data files GeoTIFFs?

[isaaccorley]

They are just dummy tiffs without georeferenced metadata. The actual files are geotiffs.

[adamjstewart]

Okay, so we just need to create real GeoTIFFs using https://github.com/microsoft/torchgeo/tree/main/tests/data#raster-data for these tests. Can you do that?

[isaaccorley]

Yes, I'll make a PR updating them.