CSpline - Fix segment function and add utils to constrain parameter calculations

[eslickj]

Fixes

There was a bug in the segment function in the parameter class. The function is just used for testing and plotting cubic splines, so it is not likely to have affected anything. It's fixed and tests are added that would catch similar future issues.

Summary/Motivation:

This fixes a bug and adds parameter calculation options that allow you to impose certain properties on the cubic spline.

Changes proposed in this PR:

• Fix bug in segments method in CsplineParameters class (use for plotting and testing)
• Add option to extend the curve to extrapolate linearly from the endpoints using endpoint slope
• Add function to add constraints to parameter calculation problem to encourage the cspline to be increasing
• Add function to add constraints to parameter calculation problem to encourage the cspline to be decreasing
• Add function to add constraints to parameter calculation problem to require the function to be convex
• Add function to add constraints to parameter calculation problem to require the function to be concave

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