implemented Kahan summation algorithm for adding process noise to delta

angle- and delta velocity bias variance

- the contribution of process noise per iteration for these states can be so
small that it gets lost if using standard floating point summation

Signed-off-by: Roman <bapstroman@gmail.com>

EKF/ekf_helper.cpp

@@ -1722,3 +1722,11 @@ void Ekf::save_mag_cov_data()
 		}
 	}
 }
+
+float Ekf::kahanSummation(float sum_previous, float input, float &accumulator) const
+{
+	float y = input - accumulator;
+	float t = sum_previous + y;
+	accumulator = (t - sum_previous) - y;
+	return t;
+}