Images saving

TODO:

• analyze the problem
  • assess if today's choice with SCAN_SAVING.images_* params is ok
  • do we want to extend SCAN_SAVING to be able to specify file format, frames per file by camera ?
  • or do we want to use the camera object ?
  • remove save_images keyword arg from Scan ?
  • think about MCAs: another kind of device which is saving data files, need a generic way
  • compatibility with data policy: the solution need to be adapted to external saving and data policy
    • always use relative path (= use a directory under base_path) for images saving ?
• put HDF5 saving by default
• save directory should still be managed by SCAN_SAVING

Proposal by E.P and J.M:

• attach saving parameters to the camera object ?
  • like: frames per file, saving format

Let's not expose saving mode (AUTO_FRAME, etc) - like it is now.