ENH: Add mp_context plugin arg for MultiProc

[effigies]

Summary

For Python 3.7+, concurrent.futures.ProcessPoolExecutor can take initialization arguments and a multiprocessing context. This will allow Python 3.7 users to recover some of the lost flexibility of LegacyMultiProc.

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

• Add mp_context PluginArg for MultiProc.
• Restore initialization arguments

Acknowledgment

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

[codecov-io]

Codecov Report

Merging #2778 into master will decrease coverage by <.01%.
The diff coverage is 85.71%.

@@            Coverage Diff            @@
##           master   #2778      +/-   ##
=========================================
- Coverage    67.5%   67.5%   -0.01%     
=========================================
  Files         340     340              
  Lines       43271   43275       +4     
  Branches     5364    5364              
=========================================
+ Hits        29211   29213       +2     
- Misses      13360   13361       +1     
- Partials      700     701       +1

Flag	Coverage Δ
#smoketests	50.6% <85.71%> (ø)	⬆️
#unittests	64.91% <85.71%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
nipype/pipeline/plugins/multiproc.py	84.56% <85.71%> (-0.25%)	⬇️
nipype/testing/utils.py	89.65% <0%> (-1.73%)	⬇️

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[oesteban]

Very much needed. LGTM