Changes to dose grid Offset vector

This MR ads some conditions where the offset vector may be inverted/changed.

I am still not entirely convinced that this is the "proper" way of doing things, but this does align with a few DICOMs that I have access to.

In the case of DICOMs with Feet First as patient position, dose grids were upside-down (along z) and unreadable by dicompiler.