Update OAV for hardware triggering - StandardDetector OAV

The OAV device class is currently a StandardReadable and does not have any of the methods for hardware triggering/flyscanning, such as arm, disarm, wait_for_idle and prepare etc.

This should be updated so that it can be used as a detector for various experiments, and malcolm-style mapping experiments.

The only peculiarity with the OAV compared to other area-style detectors is that for hardware triggering and arming Trigger Source must be set to = Line1

The OAV is often used as a standard streaming camera between experiments so acquire should not be stopped after the experiment. Instead PV's should be returned to a state that it can freerun and PV's should be set to:

- Image Mode = Continuous
- Trigger Mode = Off
- Acquire = On
- Trigger Source = Freerun
- Capture Mode = Stream
- Capture = Off

## Acceptance Criteria
- Arm the OAV
- Accept 1, 2 and n TriggerInfo triggqers, collect data, save to hdf
- Return to streaming/viewing images and successfully end experiment