message: Remove explicit case SID_NONE
  Removing the explicit case allows for easier error handling of invalid
  structure types. In particular, you should never have an explicit
  structure record specifying no structure.
diff: @@ -35,9 +35,6 @@ Structure* StructureCatalogue::get(StructureID type)
  // derived types.
  switch (type)
  {
  - case StructureID::SID_NONE:
  - break;
  -
  case StructureID::SID_AGRIDOME:
  structure = new Agridome();
  break;
  