support parameters and make performance updates

[jalving]

This PR makes the following key changes:

• Fixes an issue with slow set_to_node_objectives. When node objectives are linear or quadratic, they now hit the add_to_expression! method which is more efficient. Nonlinear objectives are still slow, however.
• Adds support for variables constrained on creation.
• Adds support for MOI parameters
• Adds methods for checking if an objective function is separable and extract separable terms for a given optigraph
• Other minor refactoring and comments

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 53.52697% with 112 lines in your changes missing coverage. Please review.

Project coverage is 80.98%. Comparing base (8c2e276) to head (21614fc).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/utils.jl	20.00%	96 Missing ⚠️
src/node_variables.jl	87.27%	7 Missing ⚠️
src/optiedge.jl	20.00%	4 Missing ⚠️
src/optigraph.jl	90.00%	3 Missing ⚠️
src/backends/moi_backend.jl	91.66%	2 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #128      +/-   ##
==========================================
- Coverage   83.81%   80.98%   -2.84%     
==========================================
  Files          16       17       +1     
  Lines        2571     2756     +185     
==========================================
+ Hits         2155     2232      +77     
- Misses        416      524     +108     

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[jalving]

FYI @dlcole3