Export the model in Core ML format.

        Parameters
        ----------
        filename: str
          A valid filename where the model can be saved.

        Examples
        --------
        >>> model.export_coreml("MyModel.mlmodel")

def export_coreml(self, filename):
        """
        Export the model in Core ML format.

        Parameters
        ----------
        filename: str
          A valid filename where the model can be saved.

        Examples
        --------
        >>> model.export_coreml("MyModel.mlmodel")
        """
        from turicreate.toolkits import _coreml_utils
        display_name = "boosted trees classifier"
        short_description = _coreml_utils._mlmodel_short_description(display_name)
        context = {"mode" : "classification",
                   "model_type" : "boosted_trees",
                   "version": _turicreate.__version__,
                   "class": self.__class__.__name__,
                   "short_description": short_description,
                   'user_defined':{
                    'turicreate_version': _turicreate.__version__
                   }
                }
        self._export_coreml_impl(filename, context)