matrix from the 
% same class object. This is demonstrated in the System matrix (OSEM) example 
% between SA-WLS and ATP-WLS.
% Function based examples
% This is the very last example script in <https://github.com/villekf/OMEGA/blob/master/forward_backward_projections_example.m#L1137 
% forward_backward_projections_example.m>. It is no longer recommended to compute 
% the forward and/or backward projections this way.
% 
% You'll need to run the <https://github.com/villekf/OMEGA/blob/master/source/index_make