partially reverting last commit: it breaks demos/boolean/thrusections.geo

Geo/OCCEdge.cpp

@@ -191,7 +191,8 @@ bool OCCEdge::isSeam(const GFace *face) const
 
   if (face->geomType() == GEntity::CompoundSurface) return false;
   if (face->getNativeType() != GEntity::OpenCascadeModel) return false;
-  if (!face->periodic(0) && !face->periodic(1)) return false;
+  // this breaks demos/thrusections.geo:
+  //if (!face->periodic(0) && !face->periodic(1)) return false;
   bool ret;
   const TopoDS_Face *s = (TopoDS_Face*) face->getNativePtr();
   // This function was horribly slow... until ...