maintenance release

• add heterogeneous collection: the check is now on inheritance and not only on the class name of the object we try to add in a collection,
• add check on read-only and type with collections (with collections apis),
• fix bug on $db.init,
• message between systems do not invoke also the sender,
• update unit tests,
• update dev dependency,
• update readme,
• rename test folder and
• fix license.