[REF] Add gscontrol module

[tsalo]

Closes None.

Changes proposed in this pull request:

• Move gscontrol_raw and gscontrol_mmix into new gscontrol module. They are currently in model.fit and io, respectively, which are not good places for them.

[codecov]

Codecov Report

Merging #240 into master will increase coverage by 0.15%.
The diff coverage is 28.75%.

@@            Coverage Diff            @@
##           master    #240      +/-   ##
=========================================
+ Coverage   46.34%   46.5%   +0.15%     
=========================================
  Files          32      33       +1     
  Lines        2026    2032       +6     
=========================================
+ Hits          939     945       +6     
  Misses       1087    1087

Impacted Files	Coverage Δ
tedana/model/__init__.py	100% <ø> (ø)	⬆️
tedana/model/fit.py	28.03% <ø> (+0.32%)	⬆️
tedana/tests/test_gscontrol.py	100% <100%> (ø)
tedana/io.py	47.87% <100%> (+10.27%)	⬆️
tedana/gscontrol.py	22.53% <22.53%> (ø)
tedana/workflows/tedana.py	14.19% <50%> (+0.53%)	⬆️

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[tsalo]

Merging now!