AC_PosControl: add_takeoff_climb_rate method

This function simply increments the current altitude target given a
climb rate and dt

libraries/AC_AttitudeControl/AC_PosControl.cpp

@@ -169,6 +169,14 @@ void AC_PosControl::set_alt_target_from_climb_rate(float climb_rate_cms, float d
     }
 }
 
+/// add_takeoff_climb_rate - adjusts alt target up or down using a climb rate in cm/s
+///     should be called continuously (with dt set to be the expected time between calls)
+///     almost no checks are performed on the input
+void AC_PosControl::add_takeoff_climb_rate(float climb_rate_cms, float dt)
+{
+    _pos_target.z += climb_rate_cms * dt;
+}
+
 /// relax_alt_hold_controllers - set all desired and targets to measured
 void AC_PosControl::relax_alt_hold_controllers(float throttle_setting)
 {

libraries/AC_AttitudeControl/AC_PosControl.h

@@ -127,6 +127,11 @@ public:
     ///     set force_descend to true during landing to allow target to move low enough to slow the motors
     void set_alt_target_from_climb_rate(float climb_rate_cms, float dt, bool force_descend = false);
 
+    /// add_takeoff_climb_rate - adjusts alt target up or down using a climb rate in cm/s
+    ///     should be called continuously (with dt set to be the expected time between calls)
+    ///     almost no checks are performed on the input
+    void add_takeoff_climb_rate(float climb_rate_cms, float dt);
+
     /// set_alt_target_to_current_alt - set altitude target to current altitude
     void set_alt_target_to_current_alt() { _pos_target.z = _inav.get_altitude(); }