Fix in constraint modification

[mtanneau]

Closes #83

[codecov-io]

Codecov Report

Merging #84 (16990fd) into master (9f26810) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #84   +/-   ##
=======================================
  Coverage   90.32%   90.32%           
=======================================
  Files          36       36           
  Lines        2543     2543           
=======================================
  Hits         2297     2297           
  Misses        246      246           

Impacted Files	Coverage Δ
src/Interfaces/tulip_julia_api.jl	69.69% <100.00%> (ø)

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