Update dataset loading and features - Add TREC dataset
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I just copied the files that are on google storage to follow the new `_relative_data_dir ` format. It should be good to merge now :)

Well actually it seems there are some merge conflicts to fix first
2020-07-15T14:53:31Z
https://github.com/huggingface/datasets/pull/386#issuecomment-658816816
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2020-07-15T14:53:31Z
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This PR:
- add a template for a new dataset script
- update the caching structure so that the path to the cached data files is also a function of the dataset loading script hash. This way when you update a loading script the data will be automatically updated instead of falling back to the previous version (which is usually a outdated). This makes it in particular easier to iterate when writing a new dataset loading script.
- fix a bug in the `ClassLabel` feature and make it more flexible so that its methods `str2int` and `int2str` can also accept list, numpy arrays and PyTorch/TensorFlow tensors.
- add the TREC-6 dataset