AP_MotorsMulticopter: add HOVER_LEARN option to learn but not save

libraries/AP_Motors/AP_MotorsMulticopter.cpp

@@ -134,9 +134,9 @@ const AP_Param::GroupInfo AP_MotorsMulticopter::var_info[] = {
     // @Param: HOVER_LEARN
     // @DisplayName: Hover Value Learning
     // @Description: Enable/Disable automatic learning of hover throttle
-    // @Values: 0:Disabled, 1:Enabled
+    // @Values: 0:Disabled, 1:Learn, 2:LearnAndSave
     // @User: Advanced
-    AP_GROUPINFO("HOVER_LEARN", 22, AP_MotorsMulticopter, _throttle_hover_learn, 1),
+    AP_GROUPINFO("HOVER_LEARN", 22, AP_MotorsMulticopter, _throttle_hover_learn, HOVER_LEARN_AND_SAVE),
 
     AP_GROUPEND
 };
@@ -374,7 +374,7 @@ void AP_MotorsMulticopter::set_throttle_range(int16_t radio_min, int16_t radio_m
 // update the throttle input filter.  should be called at 100hz
 void AP_MotorsMulticopter::update_throttle_hover(float dt)
 {
-    if (_throttle_hover_learn > 0) {
+    if (_throttle_hover_learn != HOVER_LEARN_DISABLED) {
         _throttle_hover = _throttle_hover + (dt/(dt+AP_MOTORS_THST_HOVER_TC))*(_throttle_in-_throttle_hover);
     }
 }
@@ -570,7 +570,7 @@ void AP_MotorsMulticopter::output_motor_mask(float thrust, uint8_t mask)
 void AP_MotorsMulticopter::save_params_on_disarm()
 {
     // save hover throttle
-    if (_throttle_hover_learn > 0) {
+    if (_throttle_hover_learn == HOVER_LEARN_AND_SAVE) {
         _throttle_hover.save();
     }
 }

libraries/AP_Motors/AP_MotorsMulticopter.h

@@ -140,6 +140,13 @@ protected:
     // save parameters as part of disarming
     void save_params_on_disarm();
 
+    // enum values for HOVER_LEARN parameter
+    enum HoverLearn {
+        HOVER_LEARN_DISABLED = 0,
+        HOVER_LEARN_ONLY = 1,
+        HOVER_LEARN_AND_SAVE = 2
+    };
+
     // parameters
     AP_Int16            _yaw_headroom;          // yaw control is given at least this pwm range
     AP_Float            _thrust_curve_expo;     // curve used to linearize pwm to thrust conversion.  set to 0 for linear and 1 for second order approximation