update_v_jc: Update one element of loadings V
In krisrs1128/expPCA: Exponential Family PCA

Update one element of loadings V

1	update_v_jc(obj_fun, obj_grad_v, x_j, v_jc, a_c, s_j, lambda, mu0)

`obj_fun`	Version of loss given by bregman divergence between data and link to use in current updates.
`obj_grad_v`	Gradient of the provided loss, with respect to v.
`x_j`	The j^th dimension of X.
`v_jc`	The initial value of v_jc to start the search from.
`a_c`	The c^th component of A.
`s_j`	The s^th dimension of the matrix AV^T - a_cv_c^T.
`lambda`	The regularization parameter in the optimization.
`mu0`	The value to regularize towards.