Multiband feature minor fix

[mpaillassa]

Here is the small change @aboucaud suggested during our last meeting.

[codecov]

Codecov Report

Merging #325 (5457bf2) into develop (0866e46) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #325      +/-   ##
===========================================
+ Coverage    87.59%   87.67%   +0.07%     
===========================================
  Files           11       11              
  Lines         1532     1533       +1     
===========================================
+ Hits          1342     1344       +2     
+ Misses         190      189       -1     

Flag	Coverage Δ
unittests	87.67% <100.00%> (+0.07%)	⬆️

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Impacted Files	Coverage Δ
btk/draw_blends.py	93.82% <100.00%> (+0.02%)	⬆️
btk/multiprocess.py	100.00% <0.00%> (+3.84%)	⬆️

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[thuiop]

Looks good, I'm merging.