Fix class hierarchy for Project, it should not be an Order, as it is a Node.

Inherit from XMLMatrix, it can be required for project implementations to use matrix for project.
Create INode interface and associate it to common node classes.
Update a few docstrings.



git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@32317 20353a03-c40f-0410-a6d1-a30d3c3de9de