Hilbert type space filling curve

[Ruibin-Liu]

As shown in the README example and the Google Colab example, we can construct Z-order or Morton space filling curve patterns. In my original plan for presenting examples, I thought I should construct Hilbert-type space filling curves first. In fact, a Hilbert.py file is already in the examples folder.

The question is, how should I modify the algorithm so that we can construct Hilbert-type space filling curves?

Thoughts:

1. Change the sort order when switching to a new direction.
2. For implementation details, I can either create a new function similar to sort_divide but for Hilbert-type pattern sorting/indexing or keep the same function but change the behavior when a key word like pattern_type is specified to either Morton or Hilbert.

[Ruibin-Liu]

Turns out the above thoughts are too naive. We need to modify a lot of the sorting scheme according to https://en.wikipedia.org/wiki/Hilbert_curve#/media/File:Hilbert_curve_production_rules!.svg Because of that, we can create a different _sort_divide function for Hilbert-type curves.

[Ruibin-Liu]

For now, 2D case has been implemented, but the 3D case need much more thoughts. Read this paper: https://arxiv.org/abs/1109.2323