`sort_individuals` should return map of old->new

[hyanwong]

It is important for multi-chromosome work to be able to create multiple tree sequences that share an individuals table. Part of this is concatenating and sorting various individuals tables, so returning the mapping when sorting seems like a reasonable thing to do. It seems sensible to match the map_nodes option from simplify, and return a mapping from old to new.

Currently tables.sort_individuals() doesn't return anything, so I don't know if would be considered breaking to make it return something by default, or whether we have a "map_indivduals" parameter which then returns the map?