First pass: Node/Workflow architecture

[effigies]

This is just to get things started. Documentation is utterly untouched. Workflow is only sketched.

Based on nipype/pipeline/engine/{base,nodes}.py and https://docs.google.com/document/d/1rGMBAzgrdo3vuAmYB8PlLfaxMzGTuFv9zacI1BJIqp8/edit

cc @mgxd @djarecka

[codecov-io]

Codecov Report

Merging #1843 into enh/newengine will decrease coverage by -34.26%.
The diff coverage is n/a.

@@                Coverage Diff                 @@
##           enh/newengine    #1843       +/-   ##
==================================================
- Coverage           73.1%   38.84%   -34.26%     
==================================================
  Files               1064     1064               
  Lines              53322    53322               
==================================================
- Hits               38980    20712    -18268     
- Misses             14342    32610    +18268

Flag	Coverage Δ
#unittests	38.84% <ø> (-34.26%)	❌

Impacted Files	Coverage Δ
...pype/workflows/dmri/camino/connectivity_mapping.py	7.51% <0%> (-92.49%)	❌
nipype/workflows/dmri/mrtrix/diffusion.py	6.32% <0%> (-92.41%)	❌
...pype/workflows/dmri/mrtrix/connectivity_mapping.py	7.76% <0%> (-92.24%)	❌
nipype/interfaces/fsl/tests/test_preprocess.py	9.38% <0%> (-90.62%)	❌
nipype/algorithms/tests/test_modelgen.py	9.91% <0%> (-90.09%)	❌
nipype/interfaces/tests/test_base.py	7.22% <0%> (-89.63%)	❌
nipype/interfaces/fsl/tests/test_maths.py	11.06% <0%> (-88.94%)	❌
nipype/workflows/dmri/camino/diffusion.py	11.11% <0%> (-88.89%)	❌
nipype/interfaces/utility/tests/test_csv.py	13.04% <0%> (-86.96%)	❌
nipype/algorithms/tests/test_errormap.py	13.72% <0%> (-86.28%)	❌
... and 862 more

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