Observed values on the compressed timestamp grid.
Integer-step timestamps aligned with y.
Optional covariate rows aligned with y.
Optional per-observation fixed std devs aligned with y.
Expanded arrays on the unit grid, with missing steps filled in.
Expand integer-step timestamped observations to a NaN-stuffed unit grid.
This is intended for workflows such as MLE that operate on the same objective as a NaN-stuffed
dlmFitrun. Timestamp offsets must be positive integers relative to the initial timestamp.