code: double getMarginValue(final double defaultMargin) {
          double margin = model.get(BasicSceneGenerator.Margin.class);
          if (margin == AUTOMATIC)
              margin = defaultMargin;
          return margin;
      }
docs: Access the specified margin value or fallback to a provided
  default.
  
  @param defaultMargin default value if the parameter is 'automatic'
  @return margin