AP_Motors: add AP_MOTORS_DENSITY_COMP_DISABLED option

libraries/AP_Motors/AP_MotorsMulticopter.cpp

@@ -333,10 +333,12 @@ float AP_MotorsMulticopter::get_compensation_gain() const
 
     float ret = 1.0f / _lift_max;
 
+#if AP_MOTORS_DENSITY_COMP == 1
     // air density ratio is increasing in density / decreasing in altitude
     if (_air_density_ratio > 0.3f && _air_density_ratio < 1.5f) {
         ret *= 1.0f / constrain_float(_air_density_ratio,0.5f,1.25f);
     }
+#endif
     return ret;
 }
 

libraries/AP_Motors/AP_MotorsMulticopter.h

@@ -8,6 +8,10 @@
 
 #include "AP_Motors_Class.h"
 
+#ifndef AP_MOTORS_DENSITY_COMP
+#define AP_MOTORS_DENSITY_COMP 1
+#endif
+
 #define AP_MOTORS_DEFAULT_MIN_THROTTLE  130
 #define AP_MOTORS_DEFAULT_MID_THROTTLE  500
 #define AP_MOTORS_DEFAULT_MAX_THROTTLE  1000