WIP: refactoring setup_data_paths

3 public functions to extract and prepare data from openfmri:
- collect_openfmri
- fetch_glm_data
- fetch_classification_data

The two fetchers just prepare the data globed with collect_openfmri. I'm not happy yet with the output of these functions but I think it gives a good idea of what we could expect of them in the future. Eventually, the private functions should be put somewhere else. Let's not over design just yet.