Remove DataObject and DataAttribute classes
Add AbstractDataObject as superclass of DO and SDO
Separate making link from ExtRef to object/attribute and to control

Corrections to explicit links creation were also made and messages have
been uniformized

An FCDA may refer to one or several (when daName is missing) DA or BDA.
The previous links were wrong or not needed

Remove IED and LDevice references from LNode, rename LN to RefersToAnyLN
in LNode, change IED, LDevice, LN and AnyLN accordingly