closed loop management

Closes #1904

Ideas for the closed loop property on Axis objects (from #1904)

• all axes should have .closed_loop property
  • read-only
  • returns self.controller.get_closed_loop(axis)
• motor controllers can implement get_closed_loop
  • default implementation of Controller class returns None
  • otherwise, a controller has to return a ClosedLoop object for the specified axis
• ClosedLoop object has the following members (to be continued)
  • .on(), .off() methods
  • .state property
• Initialization of ClosedLoop objects ?
  • at the same time as the Axis ?
  • better start CL at controller initialization (i.e. at session startup) ?
  • in initialize_hardware_axis() ?
  • will start closed loop for the axis ?
• configuration
  • axis YAML configuration can have a new closed-loop section
    • activate: True => means it is activated automatically ?
    • ... in the future: parameters like window, settling time etc.
  • we need to have settings on those
    • ClosedLoop can be a BeaconObject ?
• all existing motor controllers have to be revised when the new code is ready (could be done in other issues)
• files reorganization ? (maybe for another issue)
  • adding a motor directory: bliss.controllers.motor
  • use __init__.py for backward compatibility
  • split of motor.py in 3
    • base.py (or controller.py)
    • calc_controller.py
    • closed_loop.py
  • longer term: move motor controllers from bliss.controllers.motors to bliss.controller.motor