Improve image api

The binary API for scan data was designed to fetch any kind of channel.

This PR allow to fetch images for some cases.

This could even be improved in the future, but for now i can progress with it.

Plus some related changes:

• get_scan_image now raises an exception if the image is missing
• scalar shape is defined as []. I think it is consistent with the blissdata provider
  • This maybe could break another part of the software, but this have to be fixed anyway
• An image can come from the channel node, not only a lima node