price calculation (look at comments in code for details).
- added a predicate sort method to generate more accurate MappingValue.
  Usefull when we have one generic Supply and one customer/supplier specific
  supply for the same resource.
- added warning about naming convention.


git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@5837 20353a03-c40f-0410-a6d1-a30d3c3de9de