Load a vtkMultiBlockDataSet from a file (extension ``.vtm`` or
        ``.vtmb``)

def _load_file(self, filename):
        """Load a vtkMultiBlockDataSet from a file (extension ``.vtm`` or
        ``.vtmb``)
        """
        filename = os.path.abspath(os.path.expanduser(filename))
        # test if file exists
        if not os.path.isfile(filename):
            raise Exception('File %s does not exist' % filename)

        # Get extension
        ext = vtki.get_ext(filename)
        # Extensions: .vtm and .vtmb

        # Select reader
        if ext in ['.vtm', '.vtmb']:
            reader = vtk.vtkXMLMultiBlockDataReader()
        else:
            raise IOError('File extension must be either "vtm" or "vtmb"')

        # Load file
        reader.SetFileName(filename)
        reader.Update()
        self.ShallowCopy(reader.GetOutput())