code: private ListDouble convertToListDouble(List<String> tokens) {
          double[] values = new double[tokens.size()];
          for (int i = 0; i < values.length; i++) {
              if (tokens.get(i).isEmpty()) {
                  values[i] = Double.NaN;
              } else {
                  values[i] = Double.parseDouble(tokens.get(i));
              }
          }
          return new ArrayDouble(values);
      }
docs: Given a list of tokens, convert them to a list of numbers.
  
  @param tokens the tokens to be converted
  @return the number list