Fix error related to nabla_grad : need to be replace by grad.

src/bvpy/vforms/plasticity.py

@@ -47,8 +47,9 @@ class PlasticForm(ElasticForm):
     def construct_form(self, u, v, sol):
         d = sol.geometric_dimension()
         Id = fe.Identity(d)
-        F_tot = Id + fe.nabla_grad(sol)
-        F_e = F_tot*fe.inv(self._parameters['F_plastic'])
+        F_tot = Id + fe.grad(sol)
+        Fg = self._parameters['F_plastic']
+        F_e = F_tot*fe.inv(Fg)
         C_e = F_e.T * F_e
         E_e = 0.5 * (C_e - Id)