new reporting source for hold/copy ratios

new reporting source for hold/copy ratios

This patch adds a new reporting source, "Hold/Copy Ratio per Bib
and Pickup Library (and Descendants)", that, for each bib
that has a hold request on it (or any of its components)
calculates the following:

 - active holds at each OU (including the OU's descendants)
 - holdable copies at each OU (and its descendants)
 - the ratio of the above two counts
 - counts and ratio across the entire consoritum

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