[actuator] Add an abstract method to deal with velocity or position control

Part of #117
[actuator] Add a property ControlType to various actuator

This property is used to decide if the computed speed controls the robot
in Velocity or in Position. The default is still Position, but it is now
easier to test each robot with different settings.

Part of #117
[testing] Add a basic test for velocity sensor

A bit touchy due to the 'Velocity' control, but I do my best to make it
reliable

Fix #336