text: Title: Should chunked data keep current chunk shape by default when saving?
  Body: If a hyperspy signal already contains chunked dask array data, it will currently not automatically save the array with those chunk shapes unless specified in the `chunks` argument to `s.save()`. Should it? I think it would be the expected behaviour to keep the chunk shape.
  
  It would be good to clarify this in the documentation, since people typically save large files.
  
  **Example code:**
  ```python
  import hyperspy.api as hs
  import dask.array as da
  
  data = da.random.random((10000, 3000))
  data = data.rechunk((100, 3000))
  s = hs.signals.Signal1D(data)
  s = s.as_lazy()
  s.data # chunks: (100, 3000)
  
  s.save('test.hspy')
  s2 = hs.load('test.hspy', lazy=True)
  s2.data # chunks: (40, 3000)
  
  s.save('test2.hspy', chunks = (100, 3000))
  s3 = hs.load('test2.hspy', lazy=True)
  s3.data # chunks: (100, 3000)
  ```