There are cases where we dot NOT want to simply sum the quantities of all the
variants. For example when dealing with manufacturing capacities, we may have
to chose between variants because we can't make ALL of them with the same
components.
So instead of a simple non-modular implementation, we'll let each module define
his own implementation of how to compute the product template's quantity
available for sale.
Conflicts:
stock_available/__openerp__.py
stock_available_immediately/__openerp__.py
