Support any Integer/Boolean/Real value in SelectExpression

[benedekh]

SelectExpression exptects an Expression as operand from which it chooses a value. At the moment IntegerRangeLiteral and ArrayListLiteral expressions are supported. However, it would be useful if we could select any Integer, Boolean, Real, etc. values not only from a range.

/cc @vincemolnar

[grbeni]

@vincemolnar and I have consulted about the issue and we have concluded that the transformation of such select expressions to havoc expressions in the XSTS model can lead to non-deterministic behavior in choice actions due to the else branch. This could be addressed by for example introducing if-else actions in the XSTS language in the future.