Fix compatibility of ImageTransformer with Pandas 2.1.2

[JulioAPeraza]

Closes #842.

Changes proposed in this pull request:

• Add new_file to a copy of images_df instead.

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (32577f7) 89.09% compared to head (9dad264) 89.12%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #843      +/-   ##
==========================================
+ Coverage   89.09%   89.12%   +0.02%     
==========================================
  Files          48       49       +1     
  Lines        6108     6142      +34     
==========================================
+ Hits         5442     5474      +32     
- Misses        666      668       +2     

Files	Coverage Δ
nimare/workflows/__init__.py	100.00% <100.00%> (ø)
nimare/workflows/ale.py	95.12% <ø> (ø)
nimare/workflows/misc.py	100.00% <100.00%> (ø)
nimare/transforms.py	76.66% <75.00%> (ø)
nimare/diagnostics.py	97.53% <66.66%> (-1.20%)	⬇️

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[jdkent]

LGTM, thanks for noticing this @JulioAPeraza