code: public Double getDouble (String key, Supplier<Double> notFound) {
          return getParsed (key, Double::new, notFound);
      }
docs: Retrieve a mapped element and return it as a Double.
  
  @param key A string value used to index the element.
  @param notFound A function to create a new Double if the requested key was not found
  @return The element as a Double, or notFound if the element is not found.