trim excessive trailing whitespace from subfield contents

trim excessive trailing whitespace from subfield contents

If a subfield has too much (arbitrarily defined as at least 10)
of trailing whitespace, trim the whitespace.  This works around
a problem applying certain stylesheets (like the MARCXML-to-MODS
stylesheet) that use a recursive XSLT function to trim whitespace.

Note that only "excessive" whitespace is trimmed; some systems
emit subfields that contain semantically significant trailing
whitespace in certain fields.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>