Dummy BIDS data fetching

[TheChymera]

Since we're making progress on fetching, maybe it would be a good idea to add fetch capabilities for dummy BIDS data (iirc the BIDS people publish a dummy dataset). This would be important in order to test our BIDS iterator functions (which are and should continue to be the backbone of our analysis function input).

This function - if truncated to exclude the actual analysis) could easily test the input autodetection and iterator creation capabilities, and make a great test!