- added shutter init() to evaluate shutter type used eventually
  depending on camera config (frame transfer mode on frelon)
- config.get(shutter, None) instead of putting None in config
  which was not properly handled. It is returned as string
  from the config : easier to comment unwanted shutter config line

in TomoCcd: split use_shutter and has_shutter:
- has_shutter defines if the CCD has the shutter capability
- use_shutter defines if the CCD can control the shutter depending
  on its configuration