Implement Services

Implement services for some autopilot commands :
- drone_autopilot/setReference
- drone_autopilot/setMaxControl
- drone_autopilot/setInitialReachDist
- drone_autopilot/setStayWithinDist
- drone_autopilot/setStayTime
- drone_autopilot/start
- drone_autopilot/stop
- drone_autopilot/clearCommands
- drone_autopilot/hover
- drone_autopilot/lockScaleFP