Support masking backprojection

**Use cases**
- Sinogram's image shape is not rectangler
- Masking of problematic objects, like metal for MAR (metal artifact reduction)

**How should it work IMO**
A mask per sinogram's image or one for all images in the sinogram.
the mask values should not be backprojected into the volume, and the pixel should not be considered in the diff calculation

**Additional context**
mask implementation in astra: https://www.astra-toolbox.com/docs/misc.html

hope it make sense,
Thanks!

Start by reading the ASTRA masking documentation linked in the issue, then locate TIGRE's sinogram backprojection and diff-calculation entry points. Done means defining whether masks apply per sinogram image or to all images, excluding masked values from backprojection, and excluding them from difference calculations.

Written by the indexing model from the issue text.