ctDiscretiseData: Discretise long format continuous time (ctsem) data to specific timestep.
Description
Extends and rounds timing information so equal intervals, according to specified
timestep, are achieved. NA's are inserted in other columns as necessary,
any columns specified by TDpredNames or TIpredNames have zeroes rather than NA's
inserted (because some estimation routines do not tolerate NA's in covariates).