Change base dataset / results classes to reflect BIDS structure

[jsosulski]

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This could:

1. Allow datasets to have different conditions.
2. Make incorporating new BIDS datasets super easy. We could even provide an abstract BidsDataset super-class which only needs very minor implementation, i.e. download code and paths. The rest could be gathered from the standardized BIDS structure automatically.

I will provide a draft PR soon.