AP_RangeFinder: use RNGFND_OFFSET for distance offset in PWM and I2C

libraries/AP_RangeFinder/AP_RangeFinder_PX4.cpp

@@ -136,6 +136,7 @@ void AP_RangeFinder_PX4::update(void)
 
     if (count != 0) {
         state.distance_cm = sum / count * 100.0f;
+        state.distance_cm += ranger._offset[state.instance];
     }
 }
 

libraries/AP_RangeFinder/AP_RangeFinder_PX4_PWM.cpp

@@ -105,7 +105,7 @@ void AP_RangeFinder_PX4_PWM::update(void)
         _last_pulse_time_ms = now;
 
         // setup for scaling in meters per millisecond
-        float distance_cm = pwm.pulse_width * 0.1f * scaling;
+        float distance_cm = pwm.pulse_width * 0.1f * scaling + ranger._offset[state.instance];
 
         if (distance_cm > ranger._max_distance_cm[state.instance] ||
             distance_cm < ranger._min_distance_cm[state.instance]) {

libraries/AP_RangeFinder/RangeFinder.cpp

@@ -44,7 +44,7 @@ const AP_Param::GroupInfo RangeFinder::var_info[] PROGMEM = {
 
     // @Param: _OFFSET
     // @DisplayName: rangefinder offset
-    // @Description: Offset in volts for zero distance
+    // @Description: Offset in volts for zero distance for analog rangefinders. Offset added to distance in centimeters for PWM and I2C Lidars
     // @Units: Volts
     // @Increment: 0.001
     AP_GROUPINFO("_OFFSET",  3, RangeFinder, _offset[0], 0.0),