Appends stars from another StarPopulations, in place.

        :param other:
            Another :class:`StarPopulation`; must have same columns as ``self``.

def append(self, other):
        """Appends stars from another StarPopulations, in place.

        :param other:
            Another :class:`StarPopulation`; must have same columns as ``self``.

        """
        if not isinstance(other,StarPopulation):
            raise TypeError('Only StarPopulation objects can be appended to a StarPopulation.')
        if not np.all(self.stars.columns == other.stars.columns):
            raise ValueError('Two populations must have same columns to combine them.')

        if len(self.constraints) > 0:
            logging.warning('All constraints are cleared when appending another population.')

        self.stars = pd.concat((self.stars, other.stars))

        if self.orbpop is not None and other.orbpop is not None:
            self.orbpop = self.orbpop + other.orbpop