An optional named list of parameter values (beta and theta).
See details. Optionally, gamma elements can also be added, which leads to
'fixing' those random effects terms at the supplied values.
newdata
An optional data.frame of observations for which we
want to calculate the residuals.
fix_smooth
Logical; it is set to TRUE by default, if
newdata is supplied. The random effects parameters corresponding
the smooth terms are fixed and not fitted (posterior mode) to
newdata instead they are treated just like fixed effects
parameters. See details.