Create structured array from data frame.

        Parameters
        ----------
        event : object
            Identifier of column containing event indicator.
        time : object
            Identifier of column containing time.
        data : pandas.DataFrame
            Dataset.

        Returns
        -------
        y : np.array
            Structured array with two fields.

def from_dataframe(event, time, data):
        """Create structured array from data frame.

        Parameters
        ----------
        event : object
            Identifier of column containing event indicator.
        time : object
            Identifier of column containing time.
        data : pandas.DataFrame
            Dataset.

        Returns
        -------
        y : np.array
            Structured array with two fields.
        """
        if not isinstance(data, pandas.DataFrame):
            raise TypeError(
                "exepected pandas.DataFrame, but got {!r}".format(type(data)))

        return Surv.from_arrays(
            data.loc[:, event].values,
            data.loc[:, time].values,
            name_event=str(event),
            name_time=str(time))