Add __getitem__ accessor for catalogs

[smcguire-cmu]

Allows accessing catalog through square brackets by overloading __getitem__ to pass through to the Dask DataFrame and covert result to catalog if necessary.

Closes #251

[github-actions]

Before [3b245e1]	After [8180f79]	Ratio	Benchmark (Parameter)
37.1±1ms	37.8±0.5ms	1.02	benchmarks.time_kdtree_crossmatch
14.3±0.8ms	14.6±0.8ms	1.02	benchmarks.time_polygon_search
6.08±0.2ms	6.03±0.2ms	0.99	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 98.96%. Comparing base (3b245e1) to head (1fc6792).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #281   +/-   ##
=======================================
  Coverage   98.96%   98.96%           
=======================================
  Files          41       41           
  Lines        1255     1260    +5     
=======================================
+ Hits         1242     1247    +5     
  Misses         13       13           

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