RC_Channel: New method to detect if channel is in trim location

This is a new method which will return true if an RC_Channel has a PWM
value that is at its TRIM value plus or minus the allowed dead zone
around the TRIM.

libraries/RC_Channel/RC_Channel.cpp

@@ -511,3 +511,11 @@ uint16_t RC_Channel::get_limit_pwm(LimitValue limit) const
     // invalid limit value, return trim
     return radio_trim;
 }
+
+/*
+  Return true if the channel is at trim and within the DZ
+*/
+bool RC_Channel::in_trim_dz()
+{
+    return is_bounded(radio_in, radio_trim - _dead_zone, radio_trim + _dead_zone);
+}

libraries/RC_Channel/RC_Channel.h

@@ -135,6 +135,8 @@ public:
 
     static RC_Channel *rc_channel(uint8_t i);
 
+    bool in_trim_dz();
+
 private:
     AP_Int8         _reverse;
     AP_Int16        _dead_zone;