Fills in scores for forecast-target combinations that are
missing from the data, using a user-specified imputation
strategy. This is useful to ensure all models are evaluated
on the same set of targets, which avoids bias when
summarising scores.
Missing combinations are identified by comparing each value
of the compare column against the union of targets observed
across all values. The strategy is then called to fill the
metric columns for those rows.
An .imputed column is added to the output indicating which
rows were imputed (TRUE) and which are original (FALSE).