Records which either care about where they are on disk, or have other
records who care about where they are, will implement this interface.
Normally, they'll subclass PositionDependentRecordAtom or
PositionDependentRecordContainer, which will do the work of providing
the setting and updating interfaces for them.
This is a special (and dangerous) kind of Record. When created, they
need to be pinged with their current location. When written out, they
need to be given their new location, and offered the list of records
which have changed their location.