Incorrect mandatory use of exchange structures in payload

The methodology document states "those UML packages related to the exchange model being aware of – and actually using elements from – the payload content model, but not the other way round". This is inconsistent with the fact that the association from D2LogicalModel (the incorrectly - see issue #4 - named single entry point into the model) to the exchange package class Exchange is mandatory! Exchange has a mandatory compnent supplierIdentification with two mandatory attributes. Hence, no payload element can be instantiated without having an Exchange element alongside.

Suggestion: Make PayloadPublication an alternative entry point into the model. This would circumvent the mandatory use of the D2LogicalModel class. Even better: make PayloadPublication the only entry point, this might fix bug #4 as well. Future - hopefully better - exchange specifications might consider different ways to model exchange metadata, e.g. as part of a WSDL description or in a separate schema.