[2.0] Request for composability in session (possibly rig)

[dbirman]

Is your feature request related to a problem? Please describe.
Users request that we make it possible to combine two session files and possibly two rig files to make maintenance easier, i.e. given valid session_behavior.json and session_ephys.json we make it possible to do session = session_behavior + session_ephys

Describe the solution you'd like
Override __add__ to combine data streams and validate that other fields are matched

[dbirman]

Convert Stream to List[Union[]] for this