Solves using Python Interior Point Solver (PIPS).

def _solve(self, x0, A, l, u, xmin, xmax):
        """ Solves using Python Interior Point Solver (PIPS).
        """
        s = pips(self._costfcn, x0, A, l, u, xmin, xmax,
                 self._consfcn, self._hessfcn, self.opt)
        return s