code: def get_stability_criteria(self, s, n):
          
          n = get_uvec(n)
          stress = s * np.outer(n, n)
          sym_wallace = self.get_symmetric_wallace_tensor(stress)
          return np.linalg.det(sym_wallace.voigt)
docs: Gets the stability criteria from the symmetric
          Wallace tensor from an input vector and stress
          value.
  
          Args:
              s (float): Stress value at which to evaluate
                  the stability criteria
              n (3x1 array-like): direction of the applied
                  stress