Update Point Cloud Extension

[duckontheweb]

Related Issue(s):

• Update extension pydoc strings to match new extension architecture #326
• Implement collection summary functionality #327

I've intentionally left #518 for a separate PR that includes all extension name changes.

Description:

• Updates docstrings and adds Collection summaries for Point Cloud Extension

PR Checklist:

• Code is formatted (run pre-commit run --all-files)
• Tests pass (run scripts/test)
• Documentation has been updated to reflect changes, if applicable
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.

[codecov-commenter]

Codecov Report

Merging #558 (b31cd77) into main (77f203e) will increase coverage by 0.06%.
The diff coverage is 96.24%.

@@            Coverage Diff             @@
##             main     #558      +/-   ##
==========================================
+ Coverage   94.43%   94.50%   +0.06%     
==========================================
  Files          71       71              
  Lines       10611    10703      +92     
  Branches     1092     1095       +3     
==========================================
+ Hits        10021    10115      +94     
+ Misses        420      418       -2     
  Partials      170      170              

Impacted Files	Coverage Δ
pystac/extensions/pointcloud.py	95.50% <93.82%> (+0.46%)	⬆️
tests/extensions/test_pointcloud.py	100.00% <100.00%> (ø)
pystac/utils.py	95.27% <0.00%> (+1.57%)	⬆️
pystac/errors.py	95.00% <0.00%> (+5.00%)	⬆️

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