Get consistent about "title : subtitle" in mods_slim

Get consistent about "title : subtitle" in mods_slim

By teaching get_field_value() to prepend " : " to our incoming value
if the element name is "subTitle", we can ensure that records with a
title / subtitle structure have a consistent structure. Academics love
subtitles, so this was pretty glaring at Conifer.

This has a bonus effect of not appending colons after leading articles
(nonSort elements), which was annoying for "Der schutz" and the like.
Academics have a number of non-English / non-French / non-Spanish titles;
the MODS transform honours the non-filing indicator so this is a more
consistent method of generating the appropriate result.

git-svn-id: svn://svn.open-ils.org/ILS/trunk@15299 dcc99617-32d9-48b4-a31d-7c20da2025e4