`WARNING` by `ProjMatrixByBinUsingRayTracing` thrown in some notebooks

[paskino]

This happens in:

• https://github.com/SyneRBI/SIRF-Exercises/blob/master/notebooks/PET/image_creation_and_simulation.ipynb
• https://github.com/SyneRBI/SIRF-Exercises/blob/master/notebooks/PET/OSEM_reconstruction.ipynb

This can be fixed by

N = 4 # or another suitable value
acq_model.set_num_tangential_LORs(N)

However, during the training this is very confusing