HyperTreeGrid support in VTKHDF

Hey Håkon, thanks for your feedback. I did not share it but it is exactly what I’m doing in my test writer:

    descriptors = (
          # Tree 0
          1,  # Depth 1
            0, 0, 1, 0, 0, 0, 0, 0, # Depth 2
          # Tree 1
          1, # Depth 1
            0, 1, 0, 0, 0, 0, 0, 0, # Depth 2
          # Tree 3 : refined
          1,
          # Tree 4 : not refined
    )
    packed_descriptors = np.packbits(descriptors)
    root.create_dataset("Descriptors", data=packed_descriptors)

After a quick survey of bit packing techniques, using uint8 seem like a safe bet.