add nested_columns property to catalog

[dougbrn]

Closes #754

[github-actions]

Before [b0198f1] <v0.5.2>	After [7af3831]	Ratio	Benchmark (Parameter)
6.62±0s	6.61±0.01s	1	benchmarks.time_create_large_catalog
982±9ms	983±5ms	1	benchmarks.time_create_midsize_catalog
75.5±1ms	75.6±1ms	1	benchmarks.time_kdtree_crossmatch
24.9±0.6ms	24.4±0.3ms	0.98	benchmarks.time_polygon_search
13.1±0.7ms	12.5±0.09ms	0.95	benchmarks.time_box_filter_on_partition

Click here to view all benchmarks.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.32%. Comparing base (b0198f1) to head (4d978a1).
⚠️ Report is 214 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #755   +/-   ##
=======================================
  Coverage   97.31%   97.32%           
=======================================
  Files          53       53           
  Lines        2311     2314    +3     
=======================================
+ Hits         2249     2252    +3     
  Misses         62       62           

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