Add 'get_subs' 'get_ses' and other useful getters

[JoeZiminski]

datashuttle should be an easy way to run your experiment, not just collect data. There are currently not many convenience functions like getter / setter for paths. It would be really nice to do something like:

project_info = project.get_all_sub_and_ses()

for sub_info in project_info:
    for ses_path in sub_info["session_paths"]:
          
          ...

or like

shank_paths = project.find_folders("sub-001", "all_ses")
....

This wouldn't be much to implement but would expand the utility a lot, at the moment it's not really clear how to use datashuttle as a worflow manager.