Add support for 008/39

[kirkhess]

008/39 - Cataloging source    
    # - National bibliographic agency
    c - Cooperative cataloging program
    d - Other
    u - Unknown
    | - No attempt to code_

Right now the conversions have 'nac' for this byte, and the conversions default to #, which makes sense for most LC records but not for CONSER records or records with one of the other 3 codes.

Possibly bf:AdminMetadata.source.Source.code (#/c/d/u/|) would work but it probably would be better with a subclasss of Source (bf:CatalogingSource).

[kirkhess]

this commit addresses 008/39 by tying two 042 codes to 008/39 values or if the agent is dlc, but now that blank is set by 'dlc', we think the default should be 'd'? Thanks!