Add new get_combined_holdings() method to MFHD.pm

Add new get_combined_holdings() method to MFHD.pm

This commit adds a new method to the MFHD module which creates an
array of compressed holdings from all holdings for a given caption,
combining as needed.

NOTE: This method is similar to, but much less aggressive/strict than
get_compressed_holdings(). Ultimately, get_compressed_holdings()
might be deprecated in favor of this.

Signed-off-by: Dan Wells <dbw2@calvin.edu>
Signed-off-by: Lebbeous Fogle-Weekley <lebbeous@esilibrary.com>