Read an AT2 formatted time series.

        Parameters
        ----------
        filename: str
            Filename to open.

def load_at2_file(cls, filename):
        """Read an AT2 formatted time series.

        Parameters
        ----------
        filename: str
            Filename to open.

        """
        with open(filename) as fp:
            next(fp)
            description = next(fp).strip()
            next(fp)
            parts = next(fp).split()
            time_step = float(parts[1])

            accels = [float(p) for l in fp for p in l.split()]

        return cls(filename, description, time_step, accels)