Schema.org: improve MusicGroup vs. Person parsing

Take a stricter approach to defining MusicGroups instead of Persons for
the main authors of MusicAlbums. Music groups are generally catalogued
as a 110 or 710 tag, and this avoids defining birthDates and deathDates
for groups (which, however valid that might be in the real world, is not
valid for schema.org).

Signed-off-by: Dan Scott <dscott@laurentian.ca>
Signed-off-by: Ben Shum <bshum@biblio.org>