BLXV-111 BLXV-136 (8.38) Support new Merlin Threshold Sweep epics behaviour.

Untested. See notes in BLXV-136.
BLXV-137 (8.38) Restore setImageNDArrayPortInput into ADControllerBase.

This appears to have been unintentionally removed in commit cd0ca22 during
a refactor.

Without it the GDA client often resets the array plugin input to be the
camera driver. This is a problem for detectors such as the Perkin Elmer
on I15, which need processing to be performed on images before they are
displayed.
BLXV-136 (8.38) Use setImageMode rather than setTriggerMode & improve docs.
Merge remote-tracking branch 'auto+dls_sw+i15+software+gda_versions+gda_8.38b+workspace_git/8.38' into 8.38