Extract records

Would like to be able to write a cookbook entry about writing a subset of people to a GEDCOM.

Need to be able to extract records *and all associated records*, then write them out.

1. Get a list of records.
1. Get all associated records [INDI->FAM, INDI->Note, INDI->Source, INDI->OBJE, etc]
1. Do *not* include unassociated records. E.g. if extracting descendants of such-and-such person, do not include ancestors, ancestors of spouses, etc.
1. Ability to write an arbitrary list of records to a GEDCOM.

The issue names no files, tests, or entry points. Start by mapping how records and their INDI, FAM, Note, Source, and OBJE associations are represented, then inspect the GEDCOM writing path. Done means an arbitrary selected list can be written with all associated records and without unrelated records.

Written by the indexing model from the issue text.