Test if spline endpoints are at minima for a given slope cutoff.

def endpoints_minima(self, slope_cutoff=5e-3):
        """
        Test if spline endpoints are at minima for a given slope cutoff.
        """
        energies = self.energies
        try:
            sp = self.spline()
        except:
            print("Energy spline failed.")
            return None
        der = sp.derivative()
        der_energies = der(range(len(energies)))
        return {"polar": abs(der_energies[-1]) <= slope_cutoff,
                "nonpolar": abs(der_energies[0]) <= slope_cutoff}