Allow zeroth order derivative for Bezier Curve

[pepbos]

This PR changes calcValue to be a special case of calcDerivative with order zero.

This allows for less if / else switches further down the line and focuses calculations to a single function body.
Otherwise does not change the output of calcValue or calcDerivative.

Brief summary of changes

In SmoothSegmentedFunction

• calcDerivative allows order zero.
• calcValue calls calcDerivative with order zero.

Similarly, in SegmentedQuinticBezierToolkit:

• calcQuinticBezierCurveDerivU and calcQuinticBezierCurveDYDX allow for order zero.
• calcQuinticBezierCurveCurveVal calls calcQuinticBezierCurveDerivU with order zero.

Changed MustThrow unit test accordingly.

Testing I've completed

• Unit tests are still running.
• No significant change in performance (wall clock time) when simulating Rajagopal.
• No change in simulation result of Rajagopal.

CHANGELOG.md

Change too minor?

---

This change is 

[nickbianco]

One minor comment, but otherwise looks good.

@pepbos I think one reviewer for this change is fine, max two.

[nickbianco]

Should be "...greater than or equal to zero."

[pepbos]

@nickbianco thanks for reviewing :)

[nickbianco]

LGTM.