Inconsistency in API requests for associated information

[MattWindsor91]

In lists.go, there is a function GetMembers which takes a List pointer, but only uses it to get the ID to do a call for the members on that list.

In timeslot.go, there is a function GetTrackListForTimeslot which takes the timeslot ID, not a Timeslot.

This seems inconsistent---thoughts on which (or both?) we should expose? My feeling is that we should use IDs for the top-level functions and then perhaps expose members on List, Timeslot etc. which take a session as a parameter and call into those functions with the given object's ID.