Fix legacy NLP interface

[pulsipher]

This fixes a bug in the legacy NLP interface that is triggered when a nonlinear measure is in an affine/quadratic expression. Previously, this caused an error in the JuMP interface so InfiniteOpt used a try-catch hack to deal with detecting this case. Now with GenericNonlinearExpr JuMP no longer errors and actually allows mixing of NonlinearExpression and GenericNonlinearExpr (see jump-dev/JuMP.jl#3740).

This PR fixes this bug and makes v0.5 version of InfiniteOpt work with JuMP's legacy interface.

Please note that the current master branch of InfiniteOpt uses JuMP's new nonlinear API and is more performant and less hacky. It will be released with v0.6 once a few more breaking changes are finalized.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.74%. Comparing base (0961b36) to head (3f84083).

Additional details and impacted files

@@               Coverage Diff               @@
##           release-0.5     #340      +/-   ##
===============================================
- Coverage        99.77%   99.74%   -0.03%     
===============================================
  Files               36       36              
  Lines             7107     7110       +3     
===============================================
+ Hits              7091     7092       +1     
- Misses              16       18       +2     

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