TPAC: Fine-grained credits in record details

TPAC: Fine-grained credits in record details

For 7xx fields with $4 subfields containing relator codes, generate a
relator code map from http://www.loc.gov/marc/relators/relacode.html
that we can use in the record details to provide specific credits,
rather than the generic (and often inappropriate) "Added author" label.

For example:

700 1. ‡aMaley, Desmond, ‡d1954- ‡4prf.

This now generates "Maley, Desmond, 1954- (Performer)" in the record
details credit section.

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