placeName xml:ids and Barsoum corresp

Added an @xml:id to each <placeName> element, and added @corresp
attributes to Barsoum <placeName> elements to indicate their
correspondence.  Also added Pleiades <idno> to Tigris (202.xml),
replaced <state></state> with <state/> in 6.xml, revised existence dates
for 69.xml, and re-generated the html in all cases (although nothing
changed but the "Last Modified" date).