message: Cleanup function difficultyString
  Caught by Clang compiler post PR push
diff: @@ -24,8 +24,7 @@ using namespace NAS2D;
  std::string difficultyString(Difficulty difficulty)
  {
  - auto it = difficultyTable.value_comp();
  - for (auto difficultyPair : difficultyTable)
  + for (const auto& difficultyPair : difficultyTable)
  {
  if (difficultyPair.second == difficulty)
  {
  