The likelihood is implemented in libLSS/physics/likelihood/voxel_poisson.hpp. The class is named LibLSS::VoxelPoissonLikelihood.
The statistical model assumes that each voxel is independent. The probability per voxel is {$$ P(N | \lambda) = \frac{\lambda^N \mathrm{e}^{-\lambda}}{N!}. $$}