Extend pwl approximation init method to support higher initial points and single solve.

[stephenscini]

Fixes # .

Summary/Motivation:

If users want to create pwl approximations with more than 2 initial points, this was not possible in previous iteration. With ability more fine-tuned approximations, also added options for single solve support similar to global method.

Changes proposed in this PR:

• More options in pwl approx initialization method
• Added testing

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[sscini]

@michaelbynum Should be ready for review. osx/3.14 timed out if you could please rerun but all others passing. Thanks!

[blnicho]

I have one minor question but otherwise I think this looks fine.

[codecov]

Codecov Report

❌ Patch coverage is 72.00000% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.94%. Comparing base (e099f64) to head (7d3d1df).

Files with missing lines	Patch %	Lines
pyomo/devel/initialization/pwl_init.py	75.00%	6 Missing ⚠️
...evel/initialization/examples/init_polynomial_ex.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4028   +/-   ##
=======================================
  Coverage   89.94%   89.94%           
=======================================
  Files         917      917           
  Lines      109261   109281   +20     
=======================================
+ Hits        98276    98298   +22     
+ Misses      10985    10983    -2     

Flag	Coverage Δ
builders	29.12% <20.00%> (+<0.01%)	⬆️
default	86.14% <72.00%> (?)
expensive	35.10% <20.00%> (?)
linux	87.67% <72.00%> (-1.98%)	⬇️
linux_other	87.67% <72.00%> (-0.01%)	⬇️
oldsolvers	27.58% <20.00%> (+<0.01%)	⬆️
osx	83.12% <72.00%> (-0.01%)	⬇️
win	85.44% <72.00%> (+<0.01%)	⬆️

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[michaelbynum]

Looks good. I have one question.