RF: Prefer math.gcd to hand-rolled Euclid's algorithm

[effigies]

Summary

Fixes #3125.

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

Acknowledgment

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

[codecov]

Codecov Report

Merging #3177 into master will increase coverage by 0.02%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master    #3177      +/-   ##
==========================================
+ Coverage   64.85%   64.88%   +0.02%     
==========================================
  Files         299      299              
  Lines       39510    39506       -4     
  Branches     5220     5219       -1     
==========================================
+ Hits        25623    25632       +9     
+ Misses      12832    12824       -8     
+ Partials     1055     1050       -5

Flag	Coverage Δ
#unittests	64.88% <75%> (+0.02%)	⬆️

Impacted Files	Coverage Δ
nipype/algorithms/modelgen.py	59.25% <75%> (-0.36%)	⬇️
nipype/pipeline/plugins/base.py	58.9% <0%> (+1.64%)	⬆️
nipype/pipeline/plugins/legacymultiproc.py	68.81% <0%> (+3.46%)	⬆️

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[effigies]

Given that we're waiting on some responses from other PRs and an assessment of what's up with the tutorials, might as well squeeze this into 1.5.0.

Anybody have a minute for a review?

[mgxd]

+1 on removing unnecessary code

[effigies]

Thanks for the review.