Refactor some of the `Trk::GlobalChi2Fitter` ID material adding code

This commit refactors some of the code that is responsible for adding material to inner detector tracks in the Trk::GlobalChi2Fitter. It's still a complete mess, but this eases the problem a little bit. We split a large 600-odd line function into a few smaller ones, improving readability. The hope is that this will make the code easier to maintain in the future.

This MR should not affect the functional properties of the code in any way.