Update `*.gml` path in tag geometry when exporting an OGS6 object to a `*.prj` file

*.gmlfiles are copied to sim_path when exporting an OGS6 object to a *.prj file in order to execute a simulation, however, the file path in the tag <geometry> in the exported *.prj file still points to the old *.gml path.

Look at ogs6_export_sim_files().