FEM: fix unicode conversion error (sifio.py)
FEM: fix unicode conversion error (sifio.py)

The sifio module could only serialize the str type. This commits
adds support for unicode strings. Some properties of FreeCAD
objects like the equation variable name use unicode strings.

Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master