Improve recognition of PO BOX addresses

PO BOX addresses often have additional noise that prevents the geocoder from finding a good locality-level match. Analysis of rejected PO BOX addresses reveals the following pattern:

[initialGarbage] PO BOX aNumber [localityGarbage] aLocality provinceCode

Here are some examples:

PO BOX 18603 RPO LADNER HALFMOON BAY BC
PROFESSIONAL SERVICES CORPORATION PO BOX 480 ATLIN BC
RPO DUNBAR PO BOX 45033 VANCOUVER BC
PO BOX 4126 RPO SUMAS WAY ABBOTSFORD BC
PO BOX 72052 RPO OLD ORCHARD RD BURNABY BC