Faster xenium polygon parsing via ragged arrays

[LucaMarconato]

Parsing polygons for Xenium is now ~9x faster thanks to using from_ragged_array() and using plain numpy instead of pandas group_by.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 5.26316% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 37.15%. Comparing base (1431911) to head (7b73d33).

Files with missing lines	Patch %	Lines
src/spatialdata_io/readers/xenium.py	5.26%	18 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (1431911) and HEAD (7b73d33). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (1431911)	HEAD (7b73d33)
	3	2

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #365       +/-   ##
===========================================
- Coverage   62.98%   37.15%   -25.83%     
===========================================
  Files          27       27               
  Lines        3147     3154        +7     
===========================================
- Hits         1982     1172      -810     
- Misses       1165     1982      +817     

Files with missing lines	Coverage Δ
src/spatialdata_io/readers/xenium.py	18.10% <5.26%> (-54.34%)	⬇️

... and 8 files with indirect coverage changes

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