code: static DoublesUnionImpl heapifyInstance(final Memory srcMem) {
      final HeapUpdateDoublesSketch sketch = HeapUpdateDoublesSketch.heapifyInstance(srcMem);
      final DoublesUnionImpl union = new DoublesUnionImpl(sketch.getK());
      union.gadget_ = sketch;
      return union;
    }
docs: Returns a Heap DoublesUnion object that has been initialized with the data from the given
  Memory image of a DoublesSketch. The srcMem object will not be modified and a reference to
  it is not retained. The <i>maxK</i> of the resulting union will be that obtained from
  the sketch Memory image.
  
  @param srcMem a Memory image of a quantiles DoublesSketch
  @return a DoublesUnion object