This function copies the interpolation points from the
hindcast and data_obj and forecast to fit_obj.
During fit_obj, the algorithm changes
one or more of the \(y\) values of the
interpolation points in
xds_obj$data_obj$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$fit_obj$tt and xds_obj$fit_obj$yy.
The rules for hindcast_ty and forecast_ty
are setup
by setup_hindcast and setup_forecast.