A custom function to insert rows in long data with new pseudo-observations
that are being done on the specified break ages. There should be a
column called first in data with logical data that codes whether
the current row is the first for subject id. Furthermore,
the function assumes that columns age, occ,
hgt.z, wgt.z and
bmi.z are available. This function is used on the tbc
data in FIMD chapter 9. Check that out to see it in action.