Fixed spin polarized fatbands

[pfebrer]

thanks to @ialcon.

Before this commit, when computing fatbands contributions (grouping orbitals), the spin dimension was being reduced. This makes no sense because the bands for each spin might be different and therefore fatbands must be drawn separately for each spin (there is no meaningful way to aggregate them in general).

Also, there was a "bug" when filtering bands in the case of fatbands which resulted in plotting all bands even if they were out of range and slowed the rendering of the plot.

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 4 lines in your changes missing coverage. Please review.

Project coverage is 86.68%. Comparing base (af3ff99) to head (2716bda).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/sisl/viz/processors/orbital.py	78.94%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #833   +/-   ##
=======================================
  Coverage   86.68%   86.68%           
=======================================
  Files         402      402           
  Lines       51883    51886    +3     
=======================================
+ Hits        44976    44979    +3     
  Misses       6907     6907           

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[zerothi]

Thanks!