[FEM] fix missing recompute

as reported here: https://forum.freecadweb.org/viewtopic.php?p=582525#p582525

- also fix potential issue that FemGui is used by the script but not loaded

src/Mod/Fem/femtaskpanels/task_result_mechanical.py

@@ -46,6 +46,7 @@ from PySide.QtGui import QApplication
 
 import FreeCAD
 import FreeCADGui
+import FemGui
 
 import femresult.resulttools as resulttools
 
@@ -475,6 +476,9 @@ class _TaskPanel:
             maxm = max(UserDefinedFormula)
             self.update_colors_stats(UserDefinedFormula, "", minm, maxm)
 
+        # finally we must recompute the result_obj
+        self.result_obj.Document.recompute()
+
     def get_scalar_disp_list(self, vector_list, axis):
         # list is needed, as zib-object is not subscriptable in py3
         d = list(zip(*self.result_obj.DisplacementVectors))