Rover: implement set_desired_speed

Rover/Rover.cpp

@@ -214,6 +214,12 @@ bool Rover::set_desired_turn_rate_and_speed(float turn_rate, float speed)
     return true;
 }
 
+// set desired nav speed (m/s). Used for scripting.
+bool Rover::set_desired_speed(float speed)
+{
+    return control_mode->set_desired_speed(speed);
+}
+
 // get control output (for use in scripting)
 // returns true on success and control_value is set to a value in the range -1 to +1
 bool Rover::get_control_output(AP_Vehicle::ControlOutput control_output, float &control_value)

Rover/Rover.h

@@ -238,6 +238,7 @@ private:
     bool set_steering_and_throttle(float steering, float throttle) override;
     // set desired turn rate (degrees/sec) and speed (m/s). Used for scripting
     bool set_desired_turn_rate_and_speed(float turn_rate, float speed) override;
+    bool set_desired_speed(float speed) override;
     bool get_control_output(AP_Vehicle::ControlOutput control_output, float &control_value) override;
     bool nav_scripting_enable(uint8_t mode) override;
     bool nav_script_time(uint16_t &id, uint8_t &cmd, float &arg1, float &arg2) override;