Use beta maps in PermutedOLS instead of z maps

[tsalo]

Closes #714.

Changes proposed in this pull request:

• Use beta maps instead of z maps in PermutedOLS.

[codecov]

Codecov Report

Merging #715 (8147517) into main (035804c) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #715   +/-   ##
=======================================
  Coverage   85.34%   85.34%           
=======================================
  Files          41       41           
  Lines        4539     4539           
=======================================
  Hits         3874     3874           
  Misses        665      665           

Impacted Files	Coverage Δ
nimare/meta/ibma.py	99.49% <100.00%> (ø)

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