FIX: Use load_resultfile when loading a results pickle

[oesteban]

Summary

Some sections of the code were using filemanip.utils.loadpkl, which does not resolve paths (probably dodging more frequent errors from modify_paths akin to #2944).

These changes are spun off of #2971.

List of changes proposed in this PR (pull-request)

• A revision of loadpkl for readability and reliability.
• Replacement of instances where loadpkl was used for reading result files instead of pipeline.engine.utils.load_resultfile.

Acknowledgment

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

[codecov]

Codecov Report

Merging #2985 into master will decrease coverage by 3.35%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##           master    #2985      +/-   ##
==========================================
- Coverage    67.6%   64.24%   -3.36%     
==========================================
  Files         344      342       -2     
  Lines       43802    43741      -61     
  Branches     5478     5473       -5     
==========================================
- Hits        29611    28101    -1510     
- Misses      13475    14551    +1076     
- Partials      716     1089     +373

Flag	Coverage Δ
#smoketests	?
#unittests	64.24% <80%> (-0.8%)	⬇️

Impacted Files	Coverage Δ
nipype/pipeline/engine/nodes.py	77.31% <100%> (-6.79%)	⬇️
nipype/pipeline/engine/utils.py	69.62% <100%> (-10.31%)	⬇️
nipype/pipeline/plugins/base.py	59.12% <33.33%> (-1.2%)	⬇️
nipype/utils/filemanip.py	76.1% <80%> (-3.98%)	⬇️
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%)	⬇️
... and 44 more

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[effigies]

LGTM. Thanks for breaking these fixes down into digestible PRs, and sorry for the delay.

One minor suggestion, but absolutely not a big deal. I can submit a separate PR.