code: @Override
      public Double getDouble(final String key) {
          return retrieve(new Callable<Double>() {
              @Override
              public Double call() throws Exception {
                  return configuration.getDouble(key);
              }
          }, null);
      }
docs: Get a property as Double or {@literal null} if not there / or if the property is not an integer.
  
  @param key the key used in the configuration file.
  @return the property or {@literal null} if not there or property no integer