Index corporate authors based on relator codes

Index corporate authors based on relator codes

The existing stock index for corporate authors only kicks in if the
710 subfield 'e' == 'creator', but relator codes (in subfield '4') are
by many accounts a preferred mode of identifying the relationship of an
added entry. Thus, add '4' == ('aut' or 'cre') to the criteria for
indexing a corporate author.

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