make obs names unique

[melonora]

This PR should fix the issue reported here. Also added a warning of the operation being performed. We could optionally leave that out.

[LucaMarconato]

Unfortunately I don't thinks this implementation is correct because, in Visium, the obs are made unique after the old column is used to for the index of the shapes. I will change this to call obs_names_make_unique() before creating the shapes element.

Also, in Visium HD we use integers for the instance_key column, so there is no need to make the obs names unique; I will remove that.

Ideally, Visium and Visium HD should either both have integers of strings as names. But the matching between shapes and tables works in both case, and for backward compatibility it is probably better to keep things as they are now.

[LucaMarconato]

@melonora if it looks good to you please proceed with merging.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 36.10%. Comparing base (521a24d) to head (766b5b5).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #133      +/-   ##
==========================================
- Coverage   36.17%   36.10%   -0.07%     
==========================================
  Files          19       19              
  Lines        1692     1695       +3     
==========================================
  Hits          612      612              
- Misses       1080     1083       +3     

Files	Coverage Δ
src/spatialdata_io/readers/visium_hd.py	21.73% <ø> (ø)
src/spatialdata_io/readers/visium.py	20.83% <0.00%> (-0.54%)	⬇️

[giovp]

@melonora quick release note would be great thanks!

[melonora]

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