Add parameter descriptors

Instead of having the implementor install parameters at run-time, they now have
to be part of the actual class. This allows for a cleaner and more robust
implementation.
Fix conversions and use motor mixins
Rename Motor to LinearMotor
Fix quantity conversion
Use default position and velocity calibration

This includes reverting the velocity in dummy motor from q.count / q.s
to q.count
Allow dummy continuous motors to be stopped

The MRO of the mixins prevented that.
Make dummy focusing tests more robust

by not enforcing the unit to be mm in the out of limit tests
Implement state machine transitions for Parameters