Save elem.dat to file.
        The grid is saved as read in, i.e., with or without applied cutmck.
        If you want to change node coordinates, use self.nodes['raw']

        Parameters
        ----------
        filename: string
            output filename

def save_elem_file(self, output):
        """Save elem.dat to file.
        The grid is saved as read in, i.e., with or without applied cutmck.
        If you want to change node coordinates, use self.nodes['raw']

        Parameters
        ----------
        filename: string
            output filename
        """
        with open(output, 'wb') as fid:
            self._write_elem_header(fid)
            self._write_nodes(fid)
            self._write_elements(fid)
            self._write_neighbors(fid)