This function copies the interpolation points from the
hindcast and data and forecast to fitting.
During fitting, the algorithm changes
one or more of the \(y\) values of the
interpolation points in
xds_obj$data$yy. The xds_obj then calls
hindcast_ty and forecast_ty to feature xds_obj$hindcast$y (the
pre-observation interpolation points for burn-in) and xds_obj$hindcast$y
(post-observation interpolation points).
These are copied to the
full set of control points stored at
at xds_obj$fitting$tt and xds_obj$fitting$yy.
The rules for hindcast_ty and forecast_ty
are setup
by setup_hindcast and setup_forecast.