[FEM] fix direction handling in Force dialog
[FEM] fix direction handling in Force dialog

- the direction handling did not work because of yesterdays' PR from me

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