[Feature] Better handle session and run name dictionary keys

[JoeZiminski]

At present, the keys for sessions and runs in the dictionary use the user-input session and run keys, that matches the filename of the sessions and runs. Technically this (should?) be okay are they are just string, and there is no limit on the length a dictionary key can be. Nonetheless it seems error prone, maybe it is worth defining custom dictioanry key that holds both the folder name and a index, so the index is used by foldername is accessible if required.

see https://stackoverflow.com/questions/4901815/object-of-custom-type-as-dictionary-key

e.g subjects_and_sessions[0][0].save()  # save ses idx 0, run idx 0
subjects_and_sessions.keys()[0].folder_name()

maybe this could be better though.