add new parallel field modifier to extract_holdings

add new parallel field modifier to extract_holdings

Some systems embed holdings data in pairs of fields with different tags.

For example, given a record like this:

852 $h235.45 ABC$p12345
852 $h456.79 DEF$p78990
961 $t2
961 $t3

and a mapping file using the parallel modifier:

call    852 h
barcode 852 p
mattype 961 t   m:parallel

extract_holdings will extract two item records:

l_call           l_barcode    l_mattype
--------------   ----------   ---------
245.45 ABC       12345        2
456.79 DEF       78990        3

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