Support masking backprojection

[eliyash]

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!