This function repeatedly finds all ancestors and all descendants of
each individual in turn, and ensures they all have the same Family ID. Not
all connected individuals are related, e.g. all grandparents of an
individual will have the same FID, but will typically be unrelated.
When UseMaybeRel = TRUE
, probable relatives are added to existing
family clusters, or existing family clusters may be linked together.
Currently no additional family clusters are created.