@staticmethod
def line_parser(line):
    """Define a simple parser for reading the file.

        It is used in the `self.load()` to parse the input file.

        Parameters
        ----------
        line : string
            A line to parse.

        Returns
        -------
        out : tuple of ints
            out is (step number, interface number and direction).

        Note
        ----
        The interface will be subtracted '1' in the analysis.
        This is just for backwards compatibility with the old Fortran
        code.
        """
    linessplit = line.strip().split()
    try:
        step, inter = int(linessplit[0]), int(linessplit[1])
        direction = -1 if linessplit[2] == '-' else 1
        return step, inter, direction
    except IndexError:
        return None
