Add Hilbert Packing Support

[panchal]

This PR,

• Brings Hilbert Packing support on top Bulk Load.
• Allows user choice between STR and Hilbert packing techniques.

Ref: http://www.cs.cmu.edu/~christos/PUBLICATIONS.OLDER/cikm93.pdf

This was done back in 2017, but never got chance to propose here. Addresses #74.
Hope this fits the vision, let me know. Thanks.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@569cf17). Click here to learn what that means.
The diff coverage is 82.22%.

@@            Coverage Diff            @@
##             master      #98   +/-   ##
=========================================
  Coverage          ?   80.92%           
  Complexity        ?      804           
=========================================
  Files             ?       73           
  Lines             ?     2144           
  Branches          ?      290           
=========================================
  Hits              ?     1735           
  Misses            ?      321           
  Partials          ?       88

Impacted Files	Coverage Δ	Complexity Δ
...ain/java/com/github/davidmoten/rtree/PackAlgo.java	100% <100%> (ø)	1 <1> (?)
...c/main/java/com/github/davidmoten/rtree/RTree.java	94.95% <80.95%> (ø)	65 <1> (?)

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[davidmoten]

Apologies, this slipped my attention! Would you still like me to review?

[panchal]

Hello David, Yes. I would greatly appreciate it.