removed aspect project because validation was moved to the core project
as it's essential to the types and it's difficult to test the
validation, if the code is cluttered up in different projects.
Implemented a new architecture that supports extension better, and is
better suitable for the requirements of the DSL. Therefore added new
decoder classes for data units and protocol data units, which also have
unit tests associated (though not all have one yet).
added more unit tests to the core project overall.