Fix several bugs in constraints.
Bigin refactoring
oops, forget to commit a file
Fix issues and refactor
Change model of metas.
MetaAttribute is no longer an instance of engine, but is now an instance
of itself. MetaRule is now more strict : components size must be the
same between a generic and its meta.
Loose two files
Fix some tests
Add a test with singular + property constraint in reversed relations
Fix test with Meta Rule Constraint Violation Exception