ENH: Drop n_procs tag from BSplineApprox

[effigies]

BSplineApprox does not make use of multiple processors by default or accept them as an option.

[codecov-commenter]

Codecov Report

Base: 83.80% // Head: 87.71% // Increases project coverage by +3.90% 🎉

Coverage data is based on head (69c9310) compared to base (404fe3e).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #315      +/-   ##
==========================================
+ Coverage   83.80%   87.71%   +3.90%     
==========================================
  Files          24       24              
  Lines        1995     1995              
  Branches      235      307      +72     
==========================================
+ Hits         1672     1750      +78     
+ Misses        281      205      -76     
+ Partials       42       40       -2     

Flag	Coverage Δ
travis	83.80% <100.00%> (ø)
unittests	87.61% <100.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
sdcflows/workflows/fit/fieldmap.py	98.43% <100.00%> (ø)
sdcflows/workflows/fit/syn.py	69.82% <100.00%> (+21.30%)	⬆️
sdcflows/workflows/outputs.py	93.84% <0.00%> (+1.53%)	⬆️
sdcflows/interfaces/reportlets.py	89.83% <0.00%> (+1.69%)	⬆️
sdcflows/interfaces/bspline.py	80.00% <0.00%> (+3.33%)	⬆️
sdcflows/interfaces/fmap.py	87.35% <0.00%> (+14.94%)	⬆️
sdcflows/utils/phasemanip.py	94.02% <0.00%> (+28.35%)	⬆️

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[effigies]

Going to merge, but please let me know if you object before a release, @oesteban. It may be that it can go multithread in cases I don't understand.