Improve QuadraticStepSize

* Use previous step size in case interpolation is not done
* Use floats.EqualWithinRel rather than own comparison
* Use interpolation only if there is a chance of interpolating the
  function well by a quadratic
* Remove MinStepSize, it is not necessary that users manipulate it
Could you make the precursor "quadratic" instead of "default"? The default will be implied by the code, and there may eventually be many minimum step sizes.