TimeSeries of positions.

def positions(self):
        """
        TimeSeries of positions.
        """
        # if accessing and stale - update first
        if self.root.stale:
            self.root.update(self.root.now, None)

        vals = pd.DataFrame({x.name: x.positions for x in self.members
                             if isinstance(x, SecurityBase)})
        self._positions = vals
        return vals