Introduced Computer_getRemainingQuantity, This script is used to allow Customisation in how Available Items  (Computer Partitions) are accountable into Computers. By default use Simple Item API, and keep previous behaviour.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@34507 20353a03-c40f-0410-a6d1-a30d3c3de9de