Avoid Output objects for files or vector data

In my last changes to settings and Output (!71 ), I moved Outputs declaration from save_objects to App.__init__ .
Here I accidentally remove one if condition, and now we're declaring an Output object for each output parameter, even if it's not an image.

        for key in self._out_param_types:
            self.outputs[key] = Output(self, key, self._settings.get(key))

@remicress what do you think ?
Should we allow non image Output objects. What about vector data, if we have an app middle pipeline with this kind of output, that we need to flush after app init ? Or when playing with the frozen attr ?
Are we able to use something like"out.geom" or app["out.vector"].write("layer.shp"), if not there is probably no reason to keep this kind of Outputs.

And it's the same problem than App without outputs inheriting from OTBObject image functions.