[ENH] add HTML reports for each atlas / masked image

[Remi-Gau]

• uses the nilearn reports
• closes add nilearn reports in the output #66

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.00000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 89.10%. Comparing base (e303c9f) to head (f9b18ff).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #115      +/-   ##
==========================================
- Coverage   90.09%   89.10%   -1.00%     
==========================================
  Files          11       11              
  Lines         525      523       -2     
==========================================
- Hits          473      466       -7     
- Misses         52       57       +5     

Files	Coverage Δ
giga_connectome/connectome.py	94.87% <100.00%> (ø)
giga_connectome/postprocess.py	84.28% <40.00%> (-3.78%)	⬇️

... and 4 files with indirect coverage changes