mc_pos_control: migrate MPC_*_VEL_* parameter to acceleration scaling

Before #14212 the velocity control gains used in the multicopter
position controller were defined as a scale between velocity error in
one axis (or it's integral and derivative respectively) and the unit
thrust vector. The problem with this is that the normalization of the
unit thrust vector changes per vehicle or even vehicle configuration
as 0 and 100% thrust get a different physical response. That's why
the gains are now defined as scale between velocity error
(integral/derivative) and the output acceleration in m/s².

src/modules/mc_pos_control/Takeoff/Takeoff.cpp

@@ -37,11 +37,12 @@
 
 #include "Takeoff.hpp"
 #include <mathlib/mathlib.h>
+#include <lib/ecl/geo/geo.h>
 
-void Takeoff::generateInitialRampValue(const float hover_thrust, float velocity_p_gain)
+void Takeoff::generateInitialRampValue(float velocity_p_gain)
 {
 	velocity_p_gain = math::max(velocity_p_gain, 0.01f);
-	_takeoff_ramp_vz_init = -hover_thrust / velocity_p_gain;
+	_takeoff_ramp_vz_init = -CONSTANTS_ONE_G / velocity_p_gain;
 }
 
 void Takeoff::updateTakeoffState(const bool armed, const bool landed, const bool want_takeoff,

src/modules/mc_pos_control/Takeoff/Takeoff.hpp

@@ -68,7 +68,7 @@ public:
 	 * @param hover_thrust normalized thrsut value with which the vehicle hovers
 	 * @param velocity_p_gain proportional gain of the velocity controller to calculate the thrust
 	 */
-	void generateInitialRampValue(const float hover_thrust, const float velocity_p_gain);
+	void generateInitialRampValue(const float velocity_p_gain);
 
 	/**
 	 * Update the state for the takeoff.

src/modules/mc_pos_control/Takeoff/TakeoffTest.cpp

@@ -34,6 +34,7 @@
 #include <gtest/gtest.h>
 #include <Takeoff.hpp>
 #include <drivers/drv_hrt.h>
+#include <lib/ecl/geo/geo.h>
 
 TEST(TakeoffTest, Initialization)
 {
@@ -46,7 +47,7 @@ TEST(TakeoffTest, RegularTakeoffRamp)
 	Takeoff takeoff;
 	takeoff.setSpoolupTime(1.f);
 	takeoff.setTakeoffRampTime(2.0);
-	takeoff.generateInitialRampValue(.5f, 1.f);
+	takeoff.generateInitialRampValue(CONSTANTS_ONE_G / 0.5f);
 
 	// disarmed, landed, don't want takeoff
 	takeoff.updateTakeoffState(false, true, false, 1.f, false, 0);