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!