Factor out numpy-like array indexing implementation and add unit tests

[AdeelH]

Overview

This PR factors out the input-parsing for numpy-like array-indexing into a separate util function and adds unit tests for that function.

Checklist

• Added needs-backport label if PR is bug fix that applies to previous minor release
• Ran scripts/format_code and committed any changes
• Documentation updated if needed
• PR has a name that won't get you publicly shamed for vagueness

Notes

N/A

Testing Instructions

• See new unit tests.

[codecov]

Codecov Report

Merging #1765 (5004baf) into master (03eba4b) will increase coverage by 1.38%.
The diff coverage is 77.65%.

@@            Coverage Diff             @@
##           master    #1765      +/-   ##
==========================================
+ Coverage   77.98%   79.37%   +1.38%     
==========================================
  Files         186      186              
  Lines        8772     8732      -40     
==========================================
+ Hits         6841     6931      +90     
+ Misses       1931     1801     -130     

Impacted Files	Coverage Δ
rastervision_core/rastervision/core/predictor.py	20.96% <ø> (+0.96%)	⬆️
...a/label_store/semantic_segmentation_label_store.py	60.10% <18.18%> (+2.43%)	⬆️
...tervision/core/data/raster_source/raster_source.py	81.13% <33.33%> (+21.97%)	⬆️
...astervision/core/data/label_source/label_source.py	62.50% <50.00%> (+47.68%)	⬆️
...data/label_source/object_detection_label_source.py	83.33% <50.00%> (+23.05%)	⬆️
...rvision/core/data/raster_source/rasterio_source.py	80.41% <50.00%> (+11.55%)	⬆️
..._store/semantic_segmentation_label_store_config.py	80.64% <77.77%> (-6.63%)	⬇️
...ervision_core/rastervision/core/data/utils/misc.py	82.82% <97.67%> (+10.89%)	⬆️
rastervision_core/rastervision/core/__init__.py	100.00% <100.00%> (ø)
rastervision_core/rastervision/core/box.py	100.00% <100.00%> (ø)
... and 1 more

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