Clean kalman

Add some comments here and there and fix a small bug in the extrapolation of the covariance

config/config_geo_B_debug.info

@@ -9,7 +9,7 @@ B_field {
     ; simple magnetic field: uniform, constant, long y direction only
     z_min       300.  ; [cm]
     z_max       800.  ; [cm]
-    magnitude   1     ; [T]
+    magnitude   2     ; [T]
 }
 
 ; to enable/disable the multiple scattering