Character vector: Only include specified varying parameters - see fit$pars$varying.
arma
Whether to include autoregressive effects.
TRUE Compute autoregressive residuals. Requires the response variable in newdata.
FALSE Disregard the autoregressive effects. For family = gaussian(), predict() just use sigma for residuals.
nsamples
Integer or NULL. Number of samples to return/summarise.
If there are varying effects, this is the number of samples from each varying group.
NULL means "all". Ignored if both are FALSE. More samples trade speed for accuracy.