@classmethod
def parse_n_atoms(cls, filename, **kwargs):
    """Read the coordinate file and deduce the number of atoms

        Returns
        -------
        n_atoms : int
          the number of atoms in the coordinate file

        Raises
        ------
        NotImplementedError
          when the number of atoms can't be deduced
        """
    raise NotImplementedError('{} cannot deduce the number of atoms'.format
        (cls.__name__))
