Some fixes in sisl.viz

[pfebrer]

• There was a bug for selecting spin contributions where you couldn't ask for spin 0 or 1 in spin polarized data.
• Added plot index as metadata of traces for merged plots (in plotly), so that the traces from one childplot can easily be target.

[codecov]

Codecov Report

Attention: 23 lines in your changes are missing coverage. Please review.

Comparison is base (623eec8) 87.27% compared to head (898e76c) 87.25%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #619      +/-   ##
==========================================
- Coverage   87.27%   87.25%   -0.02%     
==========================================
  Files         355      355              
  Lines       47680    47719      +39     
==========================================
+ Hits        41611    41638      +27     
- Misses       6069     6081      +12     

Files	Coverage Δ
src/sisl/viz/plots/merged.py	71.42% <ø> (ø)
src/sisl/viz/processors/tests/test_orbital.py	99.29% <100.00%> (+0.03%)	⬆️
src/sisl/viz/processors/xarray.py	73.78% <100.00%> (+1.33%)	⬆️
src/sisl/viz/processors/orbital.py	89.09% <86.20%> (+2.37%)	⬆️
src/sisl/viz/figure/plotly.py	25.51% <5.00%> (-1.60%)	⬇️

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