DDSim.Gun: python3: fix problem with isotrop being set to False

this worked accidentally in python2, but now the dictionary order is fixed and isotrop was set to false when parsing the default values, because it happens after the setting of distribution now

DDG4/python/DDSim/Helper/Gun.py

@@ -70,7 +70,7 @@ class Gun(ConfigHelper):
     use the options phiMin, phiMax, thetaMin, and thetaMax to limit the range of randomly distributed directions
     if one of these options is not None the random distribution will be set to True and cannot be turned off!
     """
-    return self._isotrop
+    return self._isotrop or bool(self._distribution)
 
   @isotrop.setter
   def isotrop(self, val):