Replace pandas.DataFrame.append with pandas.concat

[JulioAPeraza]

Closes None.

Changes proposed in this pull request:
pandas.DataFrame.append is deprecated and will be removed in a future version of pandas.

• Replace pandas.DataFrame.append with pandas.concat.

[codecov]

Codecov Report

Base: 88.64% // Head: 88.64% // No change to project coverage 👍

Coverage data is based on head (ff3e257) compared to base (029c42f).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #774   +/-   ##
=======================================
  Coverage   88.64%   88.64%           
=======================================
  Files          38       38           
  Lines        4324     4324           
=======================================
  Hits         3833     3833           
  Misses        491      491           

Impacted Files	Coverage Δ
nimare/dataset.py	89.57% <100.00%> (ø)
nimare/transforms.py	75.36% <100.00%> (ø)

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[jdkent]

LGTM, I didn't see any more places where dataframe appending was being used.

[jdkent]

merging, this is a minimal change, thanks!