Discussion: How to handle beamlines with shared optics?

Brought up by @oliwenmandiamond during an athena drop in session

We should agree on and document a standard way to handle beamlines with shared optics (eg i19-1 and i19-2). We need a sensible way of protecting against one of these beamlines trying to control a device which is in use by the other beamline, probably using some kind of baton management. We should avoid a solution which requires the developer to constantly be checking against some variable, as is the case in GDA.

## Acceptance Criteria
* Solution agreed upon and documented