poisson_obj: Bregman Loss for Poisson data
In krisrs1128/expPCA: Exponential Family PCA

Bregman Loss for Poisson data

1	poisson_obj(x, a, v, s, lambda, mu0)

`x`	A single row of column of x, for which we want to minimize distance to the mean.
`a`	Either a vector or scale representing the latent scores.
`v`	Either a vector or scale representing the latent loadings.
`s`	Either the s^th row or column of the matrix AV^T - a_cv_c^T
`lambda`	The regularization parameter in the optimization.
`mu0`	The value to regularize towards.