Fix relative velocity

[gouarin]

No description provided.

[codecov-io]

Codecov Report

Merging #82 into master will increase coverage by 3.45%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #82      +/-   ##
==========================================
+ Coverage    68.8%   72.26%   +3.45%     
==========================================
  Files          44       44              
  Lines        4780     4788       +8     
==========================================
+ Hits         3289     3460     +171     
+ Misses       1491     1328     -163

Impacted Files	Coverage Δ
pylbm/algorithm/base.py	61.92% <0%> (-0.85%)	⬇️
pylbm/simulation.py	80% <0%> (+0.66%)	⬆️
pylbm/generator/ast.py	65.16% <0%> (+1.12%)	⬆️
pylbm/storage.py	81.98% <0%> (+1.24%)	⬆️
pylbm/container.py	77.08% <0%> (+10.41%)	⬆️
pylbm/generator/codegen.py	65.57% <0%> (+10.55%)	⬆️
pylbm/generator/autowrap.py	88.37% <0%> (+19.76%)	⬆️
pylbm/generator/printing/cython.py	59.43% <0%> (+40.16%)	⬆️

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