AlphaFilter: merge with PX4 implementation

I made a separate implementation of the same filter for PX4. Now
that I know it's duplicate I merge the two into one and reuse it.

EKF/ekf.cpp

@@ -48,7 +48,9 @@ bool Ekf::init(uint64_t timestamp)
 {
 	bool ret = initialise_interface(timestamp);
 	reset();
-
+	_accel_lpf.setAlpha(.1f);
+	_gyro_lpf.setAlpha(.1f);
+	_mag_lpf.setAlpha(.1f);
 	return ret;
 }