Fix: fetch specific label from gii labels

[AhmetNSimsek]

Test snippet:

import siibra
from nilearn import plotting
mp = siibra.get_map('julich 2.9', 'fsaverage')
mesh = mp.fetch('hoc1 left')
plotting.view_surf(
    surf_mesh=[mesh['verts'], mesh['faces']],
    surf_map=mesh['labels'],
    cmap=mp.get_colormap(),
    symmetric_cmap=False, colorbar=False
).open_in_browser()

[xgui3783]

re: test snippet mp is not defined anywhere?

[xgui3783]

makes much more sense to add the code snippet in a test? since you already spent effort writing it in the PR?

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 25.00000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 50.32%. Comparing base (5a0e6b3) to head (342974e).
Report is 432 commits behind head on main.

Files	Patch %	Lines
siibra/volumes/providers/gifti.py	25.00%	3 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##             main     #574       +/-   ##
===========================================
+ Coverage   36.81%   50.32%   +13.50%     
===========================================
  Files          61       69        +8     
  Lines        5421     6694     +1273     
===========================================
+ Hits         1996     3369     +1373     
+ Misses       3425     3325      -100     

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