Added test revealing problem with generation of 'hd' unary expression
Fix for problem for generation of 'hd' unary operator where the value returned was not cloned for value types (e.g. tuple)
Changed generation of sequences to use a 'VDMSeq' class provided by the library code. The cloning method of the 'VDMSeq' class makes it easier to obtain the value semantics that a VDM sequence has.