WIP: Initial prototype for replay <-> ophyd

- Created new_event(event), new_event_descriptor(event_descriptor),
  and new_header(header) functions as well as the convenience
  function update(payload_type, payload).  These four functions
  facilitate the interaction between ophyd and replay

@tacaswell don't merge this yet. It's meant to be a discussion with @dchabot regarding the ophyd <-> replay interaction