Creates an OrbitPopulation from a DataFrame.

        :param df:
            :class:`pandas.DataFrame` object.  Must contain the following
            columns: ``['M1','M2','P','ecc','mean_anomaly','obsx','obsy','obsz']``,
            i.e., as what is accessed via :attr:`OrbitPopulation.dataframe`.

        :return:
            :class:`OrbitPopulation`.

def from_df(cls, df):
        """Creates an OrbitPopulation from a DataFrame.

        :param df:
            :class:`pandas.DataFrame` object.  Must contain the following
            columns: ``['M1','M2','P','ecc','mean_anomaly','obsx','obsy','obsz']``,
            i.e., as what is accessed via :attr:`OrbitPopulation.dataframe`.

        :return:
            :class:`OrbitPopulation`.
        """
        return cls(df['M1'], df['M2'], df['P'],
                   ecc=df['ecc'], mean_anomaly=df['mean_anomaly'],
                   obsx=df['obsx'], obsy=df['obsy'], obsz=df['obsz'])