Add MultiObjectiveAttribute for interacting with multiobjective attributes

[odow]

Closes #462

[codecov]

Codecov Report

Base: 90.92% // Head: 90.86% // Decreases project coverage by -0.07% ⚠️

Coverage data is based on head (dda3692) compared to base (e205c16).
Patch coverage: 100.00% of modified lines in pull request are covered.

❗ Current head dda3692 differs from pull request most recent head 780557d. Consider uploading reports for the commit 780557d to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #501      +/-   ##
==========================================
- Coverage   90.92%   90.86%   -0.07%     
==========================================
  Files           5        5              
  Lines        2413     2397      -16     
==========================================
- Hits         2194     2178      -16     
  Misses        219      219              

Impacted Files	Coverage Δ
src/MOI_wrapper/MOI_multi_objective.jl	94.44% <100.00%> (-1.02%)	⬇️

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