ENH: Auto-derive input_names in Function

[effigies]

Found this branch sitting around. Will be glad to update (and improve) if there's interest.

Closes #1386.

[satra]

simplifying the Function node would be useful and would minimize typos. i'm also thinking of using cloudpickle to pickle the function and see if we can move nested functions more easily (but separate PR for that).

[effigies]

Circle tests pending, but ready for review.

[codecov-io]

Codecov Report

Merging #1918 into master will increase coverage by 2.36%.
The diff coverage is 88.23%.

@@            Coverage Diff             @@
##           master    #1918      +/-   ##
==========================================
+ Coverage   72.55%   74.91%   +2.36%     
==========================================
  Files        1064      873     -191     
  Lines       54170    44625    -9545     
  Branches     7810     7541     -269     
==========================================
- Hits        39303    33433    -5870     
+ Misses      13649    10177    -3472     
+ Partials     1218     1015     -203

Flag	Coverage Δ
#smoketests	74.91% <88.23%> (+2.36%)	⬆️
#unittests	73.08% <88.23%> (+3.05%)	⬆️

Impacted Files	Coverage Δ
nipype/interfaces/utility/tests/test_wrappers.py	87.03% <100%> (+0.76%)	⬆️
nipype/interfaces/utility/wrappers.py	81.81% <84.61%> (+1.2%)	⬆️
...rfaces/slicer/tests/test_auto_SlicerCommandLine.py	81.25% <0%> (-18.75%)	⬇️
nipype/interfaces/dipy/setup.py	7.14% <0%> (-17.86%)	⬇️
nipype/interfaces/camino/convert.py	61.88% <0%> (-9.94%)	⬇️
nipype/interfaces/camino/odf.py	72.72% <0%> (-6%)	⬇️
nipype/workflows/dmri/camino/group_connectivity.py	89.43% <0%> (-4.86%)	⬇️
...semtools/registration/tests/test_auto_BRAINSFit.py	81.81% <0%> (-3.9%)	⬇️
nipype/interfaces/fsl/tests/test_auto_Split.py	82.6% <0%> (-3.11%)	⬇️
nipype/interfaces/fsl/epi.py	62.73% <0%> (-2.81%)	⬇️
... and 237 more

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