Render a GPXModel as a single JSON structure.

def visit_GpxModel(self, gpx_model, *args, **kwargs):
        """Render a GPXModel as a single JSON structure."""
        result = OrderedDict()

        put_scalar = lambda name, json_name=None: self.optional_attribute_scalar(result, gpx_model, name, json_name)
        put_list = lambda name, json_name=None: self.optional_attribute_list(result, gpx_model, name, json_name)

        put_scalar('creator')
        put_scalar('metadata')
        put_list('waypoints')
        put_list('routes')
        put_list('tracks')
        put_list('extensions')

        return result