code: private static void putConvertion( HashMap<String, Double> group, String unit, double factor ) {
          UNIT_CONVERSIONS.put(unit, group );
          group.put( unit, factor );
      }
docs: Helper for creating static unit conversions.
  
  @param group the unit group like length, duration or angles. Units of one group can convert in another.
  @param unit the unit name
  @param factor the convert factor