Improve geometry-related validation in Scene and GeoJSONVectorSource and fix a bug in AoiSampler

[AdeelH]

Overview

This PR makes some fixes motivated by #1855:

• Adds a validation check for AOI polygons to ensure that they are in fact polygons and not some other type of geometry
• Makes it so that the CRS key (if present) in the GeoJSON dict read by GeoJSONVectorSource is deleted. This is to avoid discrepancies in case the GeoJSON is subsequently passed to code that does respect the CRS field (e.g. geopandas).
• Fixes compatibility issues with shapely v2.0 in AoiSampler

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

Testing Instructions

• See new unit tests.

[codecov]

Codecov Report

Merging #1856 (9109320) into master (dc23f1f) will increase coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1856      +/-   ##
==========================================
+ Coverage   80.67%   80.78%   +0.10%     
==========================================
  Files         188      188              
  Lines        9244     9260      +16     
==========================================
+ Hits         7458     7481      +23     
+ Misses       1786     1779       -7     

Files Changed	Coverage Δ
rastervision_core/rastervision/core/data/scene.py	100.00% <100.00%> (ø)
...n/core/data/vector_source/geojson_vector_source.py	100.00% <100.00%> (ø)
...ision/pytorch_learner/dataset/utils/aoi_sampler.py	97.59% <100.00%> (+8.70%)	⬆️