Intercepts data.table's [ method for tidyped objects.
After subsetting, the method checks whether the result is still a valid
pedigree (all referenced parents still present). If so, IndNum,
SireNum, and DamNum are rebuilt and the tidyped class
is preserved. If the pedigree becomes structurally incomplete (missing parent
records), the result is degraded to a plain data.table with a warning.
Column-only selections (missing core columns) also return a plain
data.table.