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.
do you have an example of such a file in openfmri or HCP?