message: Add warning if changing element attributes to 1 in gmsh reader
diff: @@ -2374,6 +2374,12 @@ void Mesh::ReadGmshMesh(std::istream &input, int &curved, int &read_gf)
  " for all curves, surfaces, or volumes in your"
  " Gmsh geometry to values which are >= 1.");
  }
  + else if (has_nonpositive_phys_domain)
  + {
  + mfem::out << "\nGmsh reader: all element attributes were zero.\n"
  + << "MFEM only supports positive element attributes.\n"
  + << "Setting element attributes to 1.\n\n";
  + }
  if (!elements_3D.empty())
  {
  