AP_RangeFinder: added get_address()

allows AP_Periph to supply sensor_id for multiple CAN rangefinders

libraries/AP_RangeFinder/AP_RangeFinder.h

@@ -144,6 +144,11 @@ public:
         return id >= RANGEFINDER_MAX_INSTANCES? Type::NONE : Type(params[id].type.get());
     }
 
+    // get rangefinder address (for AP_Periph CAN)
+    uint8_t get_address(uint8_t id) const {
+        return id >= RANGEFINDER_MAX_INSTANCES? 0 : uint8_t(params[id].address.get());
+    }
+    
     // methods to return a distance on a particular orientation from
     // any sensor which can current supply it
     uint16_t distance_cm_orient(enum Rotation orientation) const;