AST-1613 Use xtensor data arguments for ApplyBeamToXxxAndAdd
Update ApplyBeamToDataAndAdd and ApplyArrayFactorAndAdd, so they receive reference to the UTensor data structures for data and model data instead of pointers. This way, the functions can exploit XTensor indexing and we can remove the manual indexing of these 3-D arrays.
Before merging this MR, we should have a discussion on the future of XTensor. Update: Since we decided on sticking to XTensor for now, I think it would be a nice improvement.
Edited by Maik Nijhuis