Gets the yield stress for a given direction

        Args:
            n (3x1 array-like): direction for which to find the
                yield stress

def get_yield_stress(self, n):
        """
        Gets the yield stress for a given direction

        Args:
            n (3x1 array-like): direction for which to find the
                yield stress
        """
        # TODO: root finding could be more robust
        comp = root(self.get_stability_criteria, -1, args=n)
        tens = root(self.get_stability_criteria, 1, args=n)
        return (comp.x, tens.x)