[REF] Create new stats module

[tsalo]

Closes #249.

Changes proposed in this pull request:

• tedana.model.fit.get_coeffs --> tedana.stats.get_coeffs
• tedana.model.fit.computefeats2 --> tedana.stats.computefeats2
• tedana.utils.getfbounds --> tedana.stats.getfbounds
• Update corresponding tests and API documentation.

[codecov]

Codecov Report

Merging #273 into master will increase coverage by 0.26%.
The diff coverage is 78.72%.

@@            Coverage Diff             @@
##           master     #273      +/-   ##
==========================================
+ Coverage    49.1%   49.36%   +0.26%     
==========================================
  Files          37       39       +2     
  Lines        2122     2133      +11     
==========================================
+ Hits         1042     1053      +11     
  Misses       1080     1080

Impacted Files	Coverage Δ
tedana/utils.py	57.28% <ø> (-2.36%)	⬇️
tedana/tests/test_utils.py	100% <ø> (ø)	⬆️
tedana/tests/test_stats_get_coeffs.py	100% <100%> (ø)
tedana/tests/test_stats_getfbounds.py	100% <100%> (ø)
tedana/tests/test_stats_computefeats2.py	100% <100%> (ø)
tedana/io.py	48.42% <33.33%> (+0.54%)	⬆️
tedana/model/fit.py	29.47% <50%> (-9.34%)	⬇️
tedana/selection/tedpca.py	12.9% <50%> (+1.42%)	⬆️
tedana/decomposition/pca.py	16.66% <50%> (+1.17%)	⬆️
tedana/selection/tedica.py	24.8% <50%> (ø)	⬆️
... and 3 more

---

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[jbteves]

I don't know much about the guts of what this is doing, but from a refactoring perspective this looks fantastic. I think this helps clean up a lot of the affected lines of code, and it should make testing easier later as we write more tests. Great job, @tsalo !

As a comment unrelated to review approval, it seems like there are a lot of "mysterious numbers" in the code. This PR isn't the place to address that, but I'd like to know what you would think about opening a new issue to fix that. I say "mysterious" instead of the more common "magic" because my hope would be that such an issue would de-mystify their usage. I'll comment on some of those momentarily.

[jbteves]

LGTM