Improve memory management in MKDAChi2 Estimator

[tsalo]

Closes None. The changes in this PR are small, but should reduce memory usage somewhat.

Changes proposed in this pull request:

• Reorder steps in the MKDAChi2 fit process to delete large arrays as soon as we are done with them.

[codecov]

Codecov Report

Merging #638 (5e9e8b8) into main (86eda1c) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #638      +/-   ##
==========================================
+ Coverage   87.89%   87.91%   +0.01%     
==========================================
  Files          40       40              
  Lines        4337     4343       +6     
==========================================
+ Hits         3812     3818       +6     
  Misses        525      525              

Impacted Files	Coverage Δ
nimare/meta/cbma/mkda.py	96.40% <100.00%> (+0.08%)	⬆️

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