Detect VisiumHD full-res image when path is relative

[quentinblampey]

There is a minor bug in the Visium HD reader. When we provide a relative path to the data, e.g. "data/Visium_HD_Mouse_Small_Intestine", then it will search for the microscopy image inside "data/Visium_HD_Mouse_Small_Intestine/data/Visium_HD_Mouse_Small_Intestine/microscope_image".

This is because it was using path / fullres_image_file instead of simply fullres_image_file.

I also did some minor updates to use pathlib everywhere instead of a combination of pathlib and os (it makes the code more readable I think).

[codecov-commenter]

Codecov Report

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

Project coverage is 45.50%. Comparing base (cff4896) to head (35232d0).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/spatialdata_io/readers/visium_hd.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #230      +/-   ##
==========================================
- Coverage   45.50%   45.50%   -0.01%     
==========================================
  Files          22       22              
  Lines        2149     2147       -2     
==========================================
- Hits          978      977       -1     
+ Misses       1171     1170       -1     

Files with missing lines	Coverage Δ
src/spatialdata_io/readers/visium_hd.py	18.29% <0.00%> (-0.39%)	⬇️