[DOC] add documentation & doc tests to fsl preprocess

[neuroumbrage]

Adding working doc tests and updated the documentation links:

• BET
• FAST
• McFLIRT
• SUSAN (doc links only)

[codecov-io]

Codecov Report

Merging #1851 into master will decrease coverage by 0.09%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master   #1851     +/-   ##
========================================
- Coverage    73.1%     73%   -0.1%     
========================================
  Files        1064    1059      -5     
  Lines       53328   53258     -70     
========================================
- Hits        38985   38883    -102     
- Misses      14343   14375     +32

Flag	Coverage Δ
#unittests	73% <100%> (-0.1%)	❌

Impacted Files	Coverage Δ
nipype/algorithms/tests/test_auto_ComputeDVARS.py	92.85% <ø> (ø)	✅
...lgorithms/tests/test_auto_FramewiseDisplacement.py	92.85% <ø> (ø)	✅
...nterfaces/freesurfer/tests/test_auto_BBRegister.py	92.85% <ø> (ø)	✅
nipype/pipeline/engine/tests/test_engine.py	94.23% <100%> (ø)	✅
nipype/utils/tests/test_filemanip.py	80.54% <100%> (-12.67%)	❌
nipype/interfaces/fsl/preprocess.py	77.51% <100%> (ø)	✅
nipype/interfaces/fsl/tests/test_maths.py	100% <100%> (ø)	✅
nipype/init.py	65.62% <0%> (-3.13%)	❌
nipype/utils/tests/test_cmd.py	95.65% <0%> (-2.9%)	❌
... and 4 more

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[neuroumbrage]

I rebased and ran make specs. A lot of the test files are no longer changed, however, auto tests for BBRegister, ApplyXFM, and ApplyXfm are all changed. Should I commit my updated version with the changed auto tests or without?

[effigies]

Only update auto tests where they reflect changes you've made in the interfaces they're testing. I don't think you've changed any, so I'd leave all of them out.

[satra]

@neuroumbrage - would you mind closing this PR and creating a new PR. because of some github mechanics this pull-request shows a lot of changes that are already in master.

http://stackoverflow.com/questions/16306012/github-pull-request-showing-commits-that-are-already-in-target-branch