FEM: elmer, fix SIF variable output bug
FEM: elmer, fix SIF variable output bug

The property "FluxVariable" of the FluxEquation document type was
exported wrongly by sifio. This was because sifio didn't recognise
unicode as a string and exported is as an array of chars.

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