improve parametrization of objective_envelope

[exaexa]

Description

makes objective_envelope a bit more universal

Checklist

• All code is formatted with
  JuliaFormatter
  (run using JuliaFormatter; format("path/to/COBREXA.jl");)
• Tests work (try with ]test COBREXA)
• All new functions have proper docstrings
• no changes in docs

[cylon-x]

🟢  Commit b90df72 is formatted properly.

[codecov]

Codecov Report

Merging #447 (b90df72) into master (2854acd) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #447   +/-   ##
=======================================
  Coverage   89.34%   89.34%           
=======================================
  Files          61       61           
  Lines        1464     1464           
=======================================
  Hits         1308     1308           
  Misses        156      156           

Impacted Files	Coverage Δ
src/analysis/envelopes.jl	100.00% <100.00%> (ø)

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