[WIP,ENH] Add FSL first level estimator

[jdkent]

Add the first level FSL estimator

TODO:

• calculate p-values
• handle F-contrasts
• interleave F-contrast output with T-contrasts to preserve original order
• write more interface tests
• fix FILMGLS interface to grab outputs correctly
• pin an actual nipype release
• make FSL installation smaller (optional)

[codecov-commenter]

Codecov Report

Merging #365 (8c97b23) into dev (c65b7d2) will decrease coverage by 59.83%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##              dev    #365       +/-   ##
==========================================
- Coverage   64.42%   4.59%   -59.84%     
==========================================
  Files          23      24        +1     
  Lines        1816    1871       +55     
  Branches      343     340        -3     
==========================================
- Hits         1170      86     -1084     
- Misses        564    1785     +1221     
+ Partials       82       0       -82     

Impacted Files	Coverage Δ
fitlins/cli/run.py	0.00% <ø> (ø)
fitlins/interfaces/fsl.py	0.00% <0.00%> (ø)
fitlins/workflows/base.py	0.00% <0.00%> (-59.14%)	⬇️
fitlins/utils/config.py	0.00% <0.00%> (-100.00%)	⬇️
fitlins/utils/strings.py	0.00% <0.00%> (-100.00%)	⬇️
fitlins/utils/__init__.py	0.00% <0.00%> (-100.00%)	⬇️
fitlins/workflows/__init__.py	0.00% <0.00%> (-100.00%)	⬇️
fitlins/interfaces/abstract.py	0.00% <0.00%> (-100.00%)	⬇️
fitlins/interfaces/afni.py	0.00% <0.00%> (-86.72%)	⬇️
fitlins/interfaces/utils.py	0.00% <0.00%> (-83.08%)	⬇️
... and 10 more

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