update.ab: Update Alpha, Beta Parameters (Sparse)
In matthewtyler/MultiScale: Multidimensional Scaling For Binary Vote Data

The user should never call this function directly! The sparse version ignores latent variables for unobserved votes

1 2	## S3 method for class 'ab' update(Z, gamma, prior, data)

`Z`	An N \times J matrix of latent variables.
`gamma`	An N \times D matrix of actor ideal points.
`prior`	A list of priors for the parameters ab, gamma: `sigma.inv.ab` a (D+1)\times (D+1) positive definite matrix corresponding to the inverse of the prior covariance matrix for (α, β). `mu.ab` a (D+1) vector corresponding to the prior mean of (α, β). `sigma.inv.gamma` a D \times D definite matrix corresponding to the inverse of the prior covariance matrix for γ. `mu.gamma` a D vector corresponding to the prior mean of γ.
`data`	A list of data values: `Y` an N \times J matrix of \pm 1 or `NA`. `N`, `J` the dimensions of `Y`. `D` the dimensions of political conflict being modeled.