[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

❌ Patch coverage is 0% with 55 lines in your changes missing coverage. Please review.
✅ Project coverage is 4.59%. Comparing base (c65b7d2) to head (8c97b23).
⚠️ Report is 136 commits behind head on dev.

Files with missing lines	Patch %	Lines
fitlins/interfaces/fsl.py	0.00%	53 Missing ⚠️
fitlins/workflows/base.py	0.00%	2 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (c65b7d2) and HEAD (8c97b23). Click for more details.

HEAD has 5 uploads less than BASE

Flag	BASE (c65b7d2)	HEAD (8c97b23)
	8	3

Additional details and impacted files

@@            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     

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