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

❌ Patch coverage is 82.22222% with 8 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (master@569cf17). Learn more about missing BASE report.
⚠️ Report is 210 commits behind head on master.

Files with missing lines	Patch %	Lines
...c/main/java/com/github/davidmoten/rtree/RTree.java	80.95%	4 Missing and 4 partials ⚠️

Additional details and impacted files

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

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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.