UMLTP2 — TestObejctive.specification and TestRequirement.specification should be ValueSpecifications

The tag definition specification of a TestObejctive and TestRequirement is currently typed by String. Although this would most likely suffice in many situation, it prevents tools to specify other forms than plain text (e.g., structured text with formal semantics such as ETSI's TPLan). If this tag definition would be of type ValueSpecification, tools/implementers could decide to go for a more capabale representation of a 'specification' such as OpaqueExpressions or Expression. Eventually, plain text is again supported by the use of LiteralString.

Replace String attributes 'specification' with ValueSpecification

As the issue description mentiones, replacing the String-typed attribute 'specification' of TestRequirement and TestObjective with ValueSpecification widens the possibilities of how to express these information.