Stochastic Average Gradient (Ameliore) SAG, SAGA 

SAG/SAGA are already implemented [here](https://github.com/TomographicImaging/Hackathon-000-Stochastic-Algorithms/blob/main/cil/NewSubsetSumFunction.py). We need to create a PR, reviewed by everyone with unittests etc. **First step is to merge SubsetSumFunction in master.**