Flint modelization (mitigate problem on scan events)

This MR improves read of last images from Flint

• Display an error if the scan was already finished
• Fix deprecation warning from silx
• Try to avoid to reach image data when it is not needed
• Some extra-code added to mitigate problems relative to frame number provided or data event sent too early

I created issues for remaining problems #1119 (wrong attribute update), #1118 (wrong available data exposed), #1069 (closed) (wrong new data event)