AP_MotorsMulticopter: add spin_when_armed_low_end_pct

returns the spin-when-armed value as a number from 0 to 1 in the range 0 to throttle_min

libraries/AP_Motors/AP_MotorsMulticopter.h

@@ -140,6 +140,9 @@ protected:
     // convert thrust (0~1) range back to pwm range
     int16_t             calc_thrust_to_pwm(float thrust_in) const;
 
+    // spin when armed as a percentage of the 0~1 range from 0 to throttle_min
+    float               spin_when_armed_low_end_pct() { return (float)_spin_when_armed.get() / _min_throttle; }
+
     // flag bitmask
     struct {
         uint8_t slow_start         : 1;    // 1 if slow start is active