The Stage is now implemented as a runtime-checkable Protocol.
Part of this led to the realisation that we can't create a DirectThingClient
based on a Protocol, only on a concrete Thing. While it might be
a good idea to address that in LabThings in due course, for now
we work around it by creating both a Protocol and a stub class. The stub class implements the protocol, and
can be used to declare a dependency.
I've now split the base camera code into a runtime-checkable Protocol
and a concrete base class (which is optional). This removes the requirement
for cameras to subclass `openflexure_microscope.things.camera.Camera`
and instead uses duck typing that should
work both statically and dynamically.
As part of this, I've moved the commonly-used dependencies
on the camera inside `openflexure_microscope.things.camera` for consistency, and to make it easy to update them if it changes in the future.
