Create hipscat index column constant

[camposandro]

Creates a constant for the HiPSCat index column (_hipscat_index). Relates to issue #38.

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[github-actions]

Before	After	Ratio	Method
[f811ab3]	[37fa8b7]
962±1ms	977±9ms	1.02	benchmarks.Suite.time_pixel_tree_creation
134±0.8ms	135±0.8ms	1.01	benchmarks.time_test_cone_filter_multiple_order
181±1ms	182±0.7ms	1.01	benchmarks.time_test_alignment_even_sky

Click here to view all benchmarks.