Compute correlation matrix in signal voxels

[JulioAPeraza]

Closes #888.

As documented in the issue, the correlation matrix for calculating the inflation reduction term in Stouffers, and the same matrix for showing the similarity between IBMA studies in the report, should be calculated in signal voxels.

Changes proposed in this pull request:

• Here, we calculate that matrix in the Stouffers._preprocess_input method and use the same in the report if it exists.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 95.83333% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.21%. Comparing base (34ad68d) to head (2a6debd). Report is 2 commits behind head on main.

Files	Patch %	Lines
nimare/reports/base.py	91.66%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #889      +/-   ##
==========================================
+ Coverage   88.20%   88.21%   +0.01%     
==========================================
  Files          48       48              
  Lines        6339     6353      +14     
==========================================
+ Hits         5591     5604      +13     
- Misses        748      749       +1     

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[jdkent]

LGTM, thanks @JulioAPeraza!