ENH: Avoid loading result from file when writing reports

[oesteban]

Summary

Minimize the access to the result property when writing
pre/post-execution reports.

This modification should particularly preempt #3009 (comment)

Acknowledgment

• (Mandatory) I acknowledge that this contribution will be available under the Apache 2 license.

[codecov]

Codecov Report

Merging #3024 into master will decrease coverage by 3.38%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #3024      +/-   ##
==========================================
- Coverage   67.52%   64.13%   -3.39%     
==========================================
  Files         344      342       -2     
  Lines       44045    43984      -61     
  Branches     5552     5548       -4     
==========================================
- Hits        29740    28211    -1529     
- Misses      13566    14646    +1080     
- Partials      739     1127     +388

Flag	Coverage Δ
#smoketests	?
#unittests	64.13% <66.66%> (-0.84%)	⬇️

Impacted Files	Coverage Δ
nipype/pipeline/engine/nodes.py	76.49% <100%> (-7.62%)	⬇️
nipype/pipeline/engine/utils.py	70.81% <62.5%> (-10.12%)	⬇️
nipype/interfaces/nilearn.py	40% <0%> (-56.67%)	⬇️
nipype/utils/spm_docs.py	25.92% <0%> (-44.45%)	⬇️
nipype/interfaces/freesurfer/base.py	50% <0%> (-30.51%)	⬇️
nipype/utils/logger.py	59.7% <0%> (-29.86%)	⬇️
nipype/algorithms/rapidart.py	35% <0%> (-29.42%)	⬇️
nipype/interfaces/spm/base.py	57.94% <0%> (-29.14%)	⬇️
nipype/utils/provenance.py	55.73% <0%> (-28.35%)	⬇️
nipype/interfaces/fsl/model.py	55.26% <0%> (-25.35%)	⬇️
... and 44 more

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[oesteban]

@effigies, if you could spare one minute with this I'd really appreciate it. Especially, the new additions to make Path more consistent across 3.x versions - https://github.com/nipy/nipype/pull/3024/files#diff-7d84fc0957a520f25bb02479beb87c51R75 (see also some tests that I added).

[effigies]

This seems reasonable overall. A couple comments on the Path hacks. In addition, would it be possible to tag the regions that are compensating for failures in specific Python versions with # PY34, # PY35, etc? That will make them easier to find and purge when we bump our minimum Python.