Use HermiteInterpolation instead of CubicInterpolation along optical axis

[leon-vv]

The interpolation technique used to speed up ray tracing can be improved

[leon-vv]

Implemented for radial geometry in aa242f7

Something similar should be possible for 3D.. research needed

[leon-vv]

Mhh, I think the same thing is possible in 3D, except that only the second derivatives are known (not the first derivatives). This makes the implementation considerably more sketchy, as we need to extract the first derivative using a finite difference method. The accuracy gains would then not be as obvious.

To properly implement this, we need a validation showing us:

• The interpolation accuracy, as a function of the number of samples along the optical axis
• The accuracy for a regular CubicSpline interpolation
• The accuracy once we use the information about the second derivatives (using a quintic spline interpolation)

Since this is quite a bit of work, and the ROI is not as obvious, moving this to longterm