NLv2: fix reporting numbers of nonlinear discrete variables

[jsiirola]

Fixes #3235

Summary/Motivation:

This resolves an error in how we were reporting the numbers of nonlinear discrete variables that was causing some MINLP solvers to crash. It also ensures that Integer variables whose domains are in [0, 1] are reported as being binary (matching AMPL's behavior).

These changes are covered by a new test, the baseline of which was verified against AMPL's generated NL file.

Changes proposed in this PR:

• correct reporting of nonlinear discrete variables
• report integer variables with domains in [0, 1] as binary
• add / update tests

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.40%. Comparing base (fa5d0b6) to head (ee05e18).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3238      +/-   ##
==========================================
+ Coverage   88.39%   88.40%   +0.01%     
==========================================
  Files         848      848              
  Lines       95322    95325       +3     
==========================================
+ Hits        84258    84273      +15     
+ Misses      11064    11052      -12     

Flag	Coverage Δ
linux	86.34% <100.00%> (+<0.01%)	⬆️
osx	76.19% <100.00%> (+<0.01%)	⬆️
other	86.54% <100.00%> (+0.01%)	⬆️
win	83.84% <100.00%> (+0.01%)	⬆️

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[grahamsparrow-8451]

@jsiirola Thank you very much for your speedy fix to this issue! I re-ran my original test code and can confirm that this resolves the issue and all three MINLP solvers now run successfully and report the same optimal objective value:

scip:None:nl_version_2 0.68
asl:bonmin:nl_version_2 0.68
asl:couenne:nl_version_2 0.68