Converts single-block piecewise-linear costs into linear
        polynomial.

def _pwl1_to_poly(self, generators):
        """ Converts single-block piecewise-linear costs into linear
        polynomial.
        """
        for g in generators:
            if (g.pcost_model == PW_LINEAR) and (len(g.p_cost) == 2):
                g.pwl_to_poly()

        return generators