FIX: Prevent double-collapsing of nested lists by OutputMultiObject

[effigies]

Summary

The saving/loading of results in Node causes an OutputMultiObject that looks like [[x]] to be collapsed twice into x, as opposed to the expected [x].

This PR detects collapses by performing a clone_traits(), which also collapses such traits, and wraps those values in a list.

Fixes #2670.

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

• Create a _protect_collapses helper function that mimics trait_get, but re-wraps collapsing traits in lists before returning the values.

Acknowledgment

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

[effigies]

While this seems to resolve the reading of result.outputs, the "Execution Outputs" in the report are still off...

[codecov-io]

Codecov Report

Merging #2673 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2673      +/-   ##
==========================================
- Coverage   67.62%   67.61%   -0.01%     
==========================================
  Files         340      340              
  Lines       43056    43076      +20     
  Branches     5329     5333       +4     
==========================================
+ Hits        29116    29127      +11     
- Misses      13238    13246       +8     
- Partials      702      703       +1

Flag	Coverage Δ
#smoketests	50.55% <100%> (+0.03%)	⬆️
#unittests	65.07% <100%> (-0.02%)	⬇️

Impacted Files	Coverage Δ
nipype/pipeline/engine/utils.py	82.64% <100%> (+0.39%)	⬆️
nipype/pipeline/plugins/legacymultiproc.py	55.95% <0%> (-5.36%)	⬇️

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[effigies]

@oesteban This is passing if you have a few minutes for review.

[oesteban]

I was at it right now :)

[effigies]

Thanks.