The data frame contains the following columns:
'Id' for subject ID
'time.start' and 'time.end' the range of time for which
the covariate values are valid - i.e. [time.start,time.end]
Additional columns are the time dependent covariates
All data for a single subject should be stored in consecutive rows, sorted
by time and the starting time of a row should match the ending time of the previous row