Splitting MaterialInteractor into individual functions

This MR groups the different calculations performed in the MaterialInteractor.hpp into functions that calculate a single part of the interaction. The functions are moved into the PointwiseMaterialInteraction.hpp file. This modification is a side-product of !652 (closed) in the preparation of the material interaction in the Kalman filter formalism.

The MR is a re-opening of !697 (closed) using another source branch.