Use tmpdir for memmap files instead of the NiMARE data directory

[tsalo]

Closes #569, but I still want to look into nilearn's approach to memory management. This PR reverts parts of #460, and I'm hoping there won't be problems with the NiMARE manuscript code.

This mostly spun out from #597 as I figured it would help there as well.

Changes proposed in this pull request:

• Use default temporary directory for memmap files instead of NiMARE data directory.

[codecov]

Codecov Report

Merging #599 (79204a7) into main (bdcec80) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #599      +/-   ##
==========================================
- Coverage   85.73%   85.73%   -0.01%     
==========================================
  Files          40       40              
  Lines        4382     4380       -2     
==========================================
- Hits         3757     3755       -2     
  Misses        625      625              

Impacted Files	Coverage Δ
nimare/utils.py	94.25% <100.00%> (-0.04%)	⬇️

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[tsalo]

I'm going to bite the bullet and merge this. Hopefully it won't cause problems for the Jupyter book.