DISP: Address pd warning.

[ntustison]

One way of addressing the following warning:

>>> meas = ants.label_overlap_measures(dkt, dkt_repaired)
/Users/ntustison/anaconda3/lib/python3.11/site-packages/ants/label/label_overlap_measures.py:43: SettingWithCopyWarning: 
A value is trying to be set on a copy of a slice from a DataFrame

See the caveats in the documentation: https://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#returning-a-view-versus-a-copy
  df.Label[0] = "All"

[coveralls]

coverage: 84.729% (+0.003%) from 84.726%
when pulling f59ba03 on SuppLabelWarning
into 280ff64 on master.