Display Serial Unit Contents in Catalog

Display Serial Unit Contents in Catalog

This commit is the first few pieces needed to expose the
content information of serial units to the public.  It does
the following:

1) Adds some linking information to the IDL to allow better
correlation between copies and units
2) Changes a few middle layer methods to pull in the contents info
and also to sort by the unit sort_key when available
3) Displays the contents information in the JSOPAC copy info table

Signed-off-by: Dan Wells <dbw2@calvin.edu>