add an example for objective_envelope

[exaexa]

Description

Example makes it more clear

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[codecov]

Codecov Report

Merging #441 (69340c9) into master (2df9398) will increase coverage by 0.01%.
The diff coverage is 85.71%.

❗ Current head 69340c9 differs from pull request most recent head 8a62eb6. Consider uploading reports for the commit 8a62eb6 to get more accurate results

@@            Coverage Diff             @@
##           master     #441      +/-   ##
==========================================
+ Coverage   89.47%   89.48%   +0.01%     
==========================================
  Files          59       59              
  Lines        1435     1446      +11     
==========================================
+ Hits         1284     1294      +10     
- Misses        151      152       +1     

Impacted Files	Coverage Δ
src/analysis/envelopes.jl	100.00% <ø> (ø)
src/base/utils/Reaction.jl	97.36% <83.33%> (-2.64%)	⬇️
src/base/types/SBMLModel.jl	85.71% <85.71%> (-0.34%)	⬇️
src/reconstruction/community.jl	91.72% <100.00%> (+0.62%)	⬆️

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[cylon-x]

🟢  Commit 69340c9 is formatted properly.

[cylon-x]

🟢  Commit 8a62eb6 is formatted properly.