[FIX,FEATURE] TransformationModelInterpolated readded

First of all, this pull request is (most likely) not yet ready but to enable a discussion directly in the code, I already opened it. Please note: This pull request does not yet re-enable the models everywhere in OpenMS.

This pull request re-adds the, during the gsl removal accidentally removed, interpolation models for TransformationModelInterpolated. The polynomial model wasn't re-added since even the gsl guys say

> Polynomial interpolation. This method should only be used for interpolating small numbers of points because polynomial interpolation introduces large oscillations, even for well-behaved datasets...

I further cleaned up and restructured the code and switch the cspline interpolation to the new CubicSpline implementation. 

The individual interpolators are hidden. If we have a general use for them I could also add them to `MATH/MISC`
delete spline_;