Real to interval

[danielward27]

Introduces some breaking changes to tidy up RationalQuadraticSpline:

• Uses RealToIncreasingOnInterval in paramax, which is simpler than the approach previously used.

Key breaking change:

• When constructing a rational quadratic spline, softmax_adjust: float | int = 1e-2, argument has been removed. Instead, minimum widths of the bins of the spline are controlled by a new min_width argument.
• As a result of the changed parameterization, results may change somewhat.

Smaller breaking changes:

• Removed the RationalQuadraticSpline.derivative method and RationalQuadraticSpline.min_deriv attributes.