code: public static MutableDoubleTuple add(
          DoubleTuple t0, DoubleTuple t1, MutableDoubleTuple result)
      {
          return DoubleTupleFunctions.apply(
              t0, t1, (a,b)->(a+b), result);
      }
docs: Add the given input tuples, and store the result in
  the given result tuple.
  
  @param t0 The first input tuple
  @param t1 The second input tuple
  @param result The tuple that will store the result
  @return The result tuple
  @throws IllegalArgumentException If the given tuples do not
  have the same {@link Tuple#getSize() size}