Add ETCI2021 Dataset

[isaaccorley]

• Adds torchgeo.datasets.ETCI2021 from the ETCI 2021 Flood Detection Challenge
• Updated docs
• Added tests/datasets/test_etci2021.py
• Added sample data for tests under tests/data/etci2021/

Questions:

• If the dataset splits (train/val/test) can be downloaded as separate files from google drive (train.zip, val.zip, test.zip), should we only download the file for the desired split or should we download the entire dataset?
• The dataset has 2 masks (water body and flood). For the train/val splits I load them into a mask of shape (2, h, w). However, the test set flood masks were never released so the water mask is loaded (h, w). Does this seem right or should I load add the additional dimension (1, h, w) for consistency?

Closes #118

[adamjstewart]

should we only download the file for the desired split or should we download the entire dataset?

Just that split

Does this seem right or should I load add the additional dimension (1, h, w) for consistency?

Prob (1, h w)

[isaaccorley]

I refactored the filenames, urls, md5s, and directories into a metadata attribute which can be indexed by the split. Let me know if you have a more standardized approach to this already. Also updated per your suggestions.

[adamjstewart]

Mostly looks good, just had a few minor comments.

[adamjstewart]

Looks like there are conflicts preventing this from being rebased and merged. Personally I never use merge commits for this very reason. I always rebase manually to resolve merge conflicts. For now I'll squash and merge.

[adamjstewart]

@isaaccorley is this strictly needed? It's currently not covered by our unit tests and I'm wondering if we can just remove it.