Remove multiprocessing flag

[thuiop]

Following @aboucaud remark on another PR, I removed the multiprocessing flag which is redundant with the cpu number. Closes #120.

[codecov]

Codecov Report

Merging #123 (26c7386) into main (43024df) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #123      +/-   ##
==========================================
- Coverage   76.12%   76.07%   -0.05%     
==========================================
  Files          10       10              
  Lines         980      978       -2     
==========================================
- Hits          746      744       -2     
  Misses        234      234              

Flag	Coverage Δ
unittests	76.07% <100.00%> (-0.05%)	⬇️

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Impacted Files	Coverage Δ
btk/draw_blends.py	94.14% <ø> (-0.04%)	⬇️
btk/measure.py	70.00% <ø> (-0.38%)	⬇️
btk/multiprocess.py	100.00% <100.00%> (ø)

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[ismael-mendoza]

Thanks @thuiop , looks great

[aboucaud]

LGTM