Builds a `HelixPair` using the defined attributes.

def build(self):
        """Builds a `HelixPair` using the defined attributes."""
        for i in range(2):
            self._molecules.append(
                self.make_helix(self.aas[i], self.axis_distances[i],
                                self.z_shifts[i], self.phis[i], self.splays[i],
                                self.off_plane[i]))
        return