Family for use with gam implementing smooth multivariate Gaussian regression.
The means for each dimension are given by a separate linear predictor, which may contain smooth components. The
Choleski factor of the response precision matrix is estimated as part of fitting.
The dimension of the response (>1).
The response is d dimensional multivariate normal, where the covariance matrix is estimated,
and the means for each dimension have sperate linear predictors. Model sepcification is via a list of gam like
formulae - one for each dimension. See example.
Currently the family ignores any prior weights, and is implemented using first derivative information sufficient for BFGS estimation of smoothing parameters. "response" residuals give raw residuals, while "deviance"
residuals are standardized to be approximately independent standard normal if all is well.