code: public String getWorldGroupName(String world) {
          String result = DEFAULT_GROUP_NAME;
          for (Entry<String, WorldGroup> entry : list.entrySet()) {
              if (entry.getValue().worldExist(world)) {
                  result = entry.getKey();
              }
          }
          return result;
      }
docs: Retrieve the name of the worldgroup a world belongs to
  
  @param world The world name
  @return The worldgroup name linked to this world