Multi-Objective Support in InfiniteOpt

[PetiteChouette]

Added support for multiple objectives within InfiniteOpt by modifying the previous support for scalars to vectors of scalars. Modified the relevant doc strings and added a subsection in the objective Markdown file.

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.75%. Comparing base (40c897d) to head (c1c70a0).

Files with missing lines	Patch %	Lines
src/TranscriptionOpt/transcribe.jl	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #434      +/-   ##
==========================================
- Coverage   99.77%   99.75%   -0.03%     
==========================================
  Files          39       39              
  Lines        6797     6810      +13     
==========================================
+ Hits         6782     6793      +11     
- Misses         15       17       +2     

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[pulsipher]

Thanks for this. A few things:

• The rocket example should be added/altered in a separate PR
• The documentation on multi-objectives should mention something about what solvers support this (e.g., MultiObjectiveAlgorithms.jl) and perhaps even link to the JuMP docs on this.
• Please also add unit tests to test this new functionality.