[WIP] Generalize bases with linear transformation and general transformations
Created by: Zaharid
Stretch the concept of basis a bit further to add non linear transformations of the grids. The change allows to easily define transformations such as ratios of two flavours that can make use of all the existing machinery for PDF grids, and particularly the PDF plots.
While this seems to work, it should be documented, tested and perhaps refined a bit.