[ekf] controlMagFusion refactor and mag field strength check (#662)

* ekf_control: Inhibit mag fusion when field magnitude is large
Move mag inhibition check in separate function

* ekf_control: pull out of functionalities out of controlMagFusion
- yaw abd mag bias observability checks
- mag 3D conditions
- load mag covariances
- set and clear mag control modes

* ekf_control: refactor mag heading/3D start/stop.
Move mag declination, mag 3d and mag heading fusion out of the main function

* ekf_control: extract mag yaw reset and mag declination fusion requirements

* ekf_control: use WMM in isStronMagneticField for mag fusion inhibition
- Correct units of WMM strength table

* ekf_control: extract mag_state_only functionality of AUTOFW (VTOL custom)
Also split inAirYawReset from onGroundYawReset

* ekf_control: extract mag automatic selection
- transform if-else into switch-case for parameter fusion type selection

* ekf_control: extract run3DMagAndDeclFusion, reorganize functions, fix
flag naming in Test script

* ekf_control: do not run mag fusion if tilt is not aligned.
Reset some variables on ground even if mag fusion is not running yet. It
could be that it runs later so we need to make sure that those variables
are properly set.

* ekf_control: move controlMagFusion and related functions to mag_control.cpp

* ekf control: check for validity of mag strength from WMM and falls back
to average earth mag field with larger gate if not valid

* ekf control: remove evyaw check for mag inhibition

* ekf control: change nested ternary operator into if-else if

* Ekf: create AlphaFilter template class for simple low-pass filtering
0.1/0.9 type low-pass filters are commonly used to smooth data, this
class is meant to abstract the computation of this filter

* ekf control: reset heading using mag_lpf data to avoid resetting on an outlier
fixes ecl issue #525

* ekf control: replace mag_states_only flag with mag_field_disturbed and
add parameter to enable or disable mag field strength check

* ekf control: remove AUTOFW mag fusion type as not needed This was implemented for VTOL but did not solve the problem and should not be used anymore

* ekf control: use start/stop mag functions everywhere instead of setting the flag

* ekf control: Run mag fusion depending on yaw_align instead of tilt_align
as there is no reason to fuse mag when the ekf isn't aligned

* AlphaFilter: add test for float and Vector3f

EKF/estimator_interface.h

@@ -44,6 +44,7 @@
 #include <ecl.h>
 #include "common.h"
 #include "RingBuffer.h"
+#include "AlphaFilter.hpp"
 
 #include <geo/geo.h>
 #include <matrix/math.hpp>
@@ -55,6 +56,8 @@ class EstimatorInterface
 {
 
 public:
+	typedef AlphaFilter<Vector3f> AlphaFilterVector3f;
+
 	EstimatorInterface() = default;
 	virtual ~EstimatorInterface() = default;
 
@@ -245,9 +248,6 @@ public:
 		_time_last_gnd_effect_on = _time_last_imu;
 	}
 
-	// set flag if only only mag states should be updated by the magnetometer
-	void set_update_mag_states_only_flag(bool update_mag_states_only) {_control_status.flags.update_mag_states_only = update_mag_states_only;}
-
 	// set air density used by the multi-rotor specific drag force fusion
 	void set_air_density(float air_density) {_air_density = air_density;}
 
@@ -272,7 +272,7 @@ public:
 	// return true if the EKF is dead reckoning the position using inertial data only
 	bool inertial_dead_reckoning() {return _is_dead_reckoning;}
 
-	virtual bool isTerrainEstimateValid() = 0;
+	virtual bool isTerrainEstimateValid() const = 0;
 	//[[deprecated("Replaced by isTerrainEstimateValid")]]
 	bool get_terrain_valid() { return isTerrainEstimateValid(); }