WIP: Resolve "Gap motor on revolver undulator"

(closes #1280 (closed))

I have added some code to the ESRF_Undulator controller:
initialize(): get rid of velocity and acceleration properties.

The property steps_per_unit should be fixed to 1 and is not necessary. A tolerance property is not necessary.

initialize_axis(): Added some code to find-out whether it is a revolver axis and whether it is disabled or not. The final axis should be initialized with the result.

enable(): A method to enable a revolver axis. The axis state needs to be handled properly. (no disable(): one undu must be always enabled)

I let the work on the Axis to the Bliss team.

Thanks