Fix optional obligatory 2 & obligatory getter

Minor fixes to prior branch (affecting Dimensions) included.
Crucial bug fix that prevented "Source"-getter from working!
Implemented final optional vs obligatory attributes in SimpleTag & DataTag & Representation.
Implemented obligatory "type" attribute in NamedEntity ctor, thus numerous instances of its' use had to be modified.
As usual all tests have been adapted to work with new versions.
Removed all tabs.
Final minor fixes to getter & setter, new "MissingAttr" exception.