This utility function takes a simulated data.frame in wide format as an input and converts it into a long format (slower compared to
Keeps all covariates that appear only once and at the first time-point constant (carry-forward).
All covariates that appear fewer than range(t) times are imputed with NA for missing time-points.
Observations with all NA's for all time-varying covariates are removed.
When removing NA's the time-varying covariates that are attributes (attnames) are not considered.
data.frame object in long format
DF.to.longDT - a faster version of
DF.to.long that uses