pub fn sparsify<T>(
    ss_free_energy_mats: &SsFreeEnergyMats<T>,
    bpp_mat: &SparseProbMat<T>,
    min_bpp: Prob
) -> SsFreeEnergyMats<T> where
    T: Unsigned + PrimInt + Hash + FromPrimitive + Integer + Ord + Sync + Send