* Added change events on std. image counter attrs. using ImageStatusCallback
* Added last_image DevEncoded DATA_ARRAY attribute, pushing change events
* Added gc command to trigger garbage collection
* Moved DATA_ARRAY generation code to _image_2_struct helper
Merge branch 'image_events' into multi_frame_read

Conflicts:
	LimaCCDs.py
* Added LimaCCDs.last_image_pushes_events boolean attr. (default=False)
  controlling if change events are generated on last_image attr. (DATA_ARRAY)