Remove hardcoding montage in plot_butterfly; add underscore to montage name in get_label_pos

[kbelgal02]

Also added corresponding tests

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.63%. Comparing base (9860b3a) to head (4f9ebb1).
⚠️ Report is 15 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #448      +/-   ##
==========================================
+ Coverage   90.32%   90.63%   +0.31%     
==========================================
  Files          27       27              
  Lines        1467     1473       +6     
==========================================
+ Hits         1325     1335      +10     
+ Misses        142      138       -4     

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[vladdez]

If i will write

    montage = get_montage("biosemi16")
    plot_butterfly(randn(length(montage.labels), 100); montage = "biosemi16")

the head disappears. Is it a bug or feature?
I think it is more bug. We need labels to extracts some channels from a montage, but if you want the full montage you kinda dont need it.