FEM: fix gmsh relative paths

Relative paths appearing in Gmsh .geo files are relative to the .geo
file, not relative to the cwd when the gmsh executable was invoked. This
is okay when using absolute paths with the default "Temporary
directories" mode, but if one selects (in Preferences -> FEM) "Beside
.FCStd file", then relative paths are written to the .geo file. This
causes a (suppressed in FreeCAD output) warning from Gmsh that the brep
file is missing as well as the error:

Unexpected error when creating mesh: File to load not existing or not readable: partname/FEMMeshGmsh/BaseFeature_Mesh.unv

In this commit we just use relative paths, which is also convenient if
users move these files elsewhere (e.g., to work directly with advanced
features in Gmsh).

src/Mod/Fem/femmesh/gmshtools.py

@@ -720,6 +720,7 @@ class GmshTools():
         self.part_obj.Shape.exportBrep(self.temp_file_geometry)
 
     def write_geo(self):
+        temp_dir = os.path.dirname(self.temp_file_geo)
         geo = open(self.temp_file_geo, "w")
         geo.write("// geo file for meshing with Gmsh meshing software created by FreeCAD\n")
         geo.write("\n")
@@ -732,7 +733,7 @@ class GmshTools():
 
         geo.write("// open brep geometry\n")
         # explicit use double quotes in geo file
-        geo.write('Merge "{}";\n'.format(self.temp_file_geometry))
+        geo.write('Merge "{}";\n'.format(os.path.relpath(self.temp_file_geometry, temp_dir)))
         geo.write("\n")
 
         # groups
@@ -879,7 +880,7 @@ class GmshTools():
         geo.write("// Ignore Physical definitions and save all elements;\n")
         geo.write("Mesh.SaveAll = 1;\n")
         # explicit use double quotes in geo file
-        geo.write('Save "{}";\n'.format(self.temp_file_mesh))
+        geo.write('Save "{}";\n'.format(os.path.relpath(self.temp_file_mesh, temp_dir)))
         geo.write("\n\n")
 
         # some useful information

src/Mod/Fem/femtest/data/elmer/group_mesh.geo

@@ -4,7 +4,7 @@
 General.NumThreads = X;
 
 // open brep geometry
-Merge "/tmp/tmp0TVZbM.brep";
+Merge "tmp0TVZbM.brep";
 
 // group data
 Physical Surface("Face1") = {1};
@@ -43,7 +43,7 @@ Coherence Mesh; // Remove duplicate vertices
 // save
 // Ignore Physical definitions and save all elements;
 Mesh.SaveAll = 1;
-Save "/tmp/tmpjVhNNb.unv";
+Save "tmpjVhNNb.unv";
 
 
 // **********************************************************************