code: def piola_kirchoff_1(self, def_grad):
          
          if not self.is_symmetric:
              raise ValueError("The stress tensor is not symmetric, \
                               PK stress is based on a symmetric stress tensor.")
          def_grad = SquareTensor(def_grad)
          return def_grad.det*np.dot(self, def_grad.inv.trans)
docs: calculates the first Piola-Kirchoff stress
  
          Args:
              def_grad (3x3 array-like): deformation gradient tensor