Preverbal Message Generation

The preverbal message generation portion of NLGen2 is working. It creates the Preverbal Message data structure, which is a set of strings representing the propositions associated with that particular preverbal message and its link structure. The link structure is currently represented as a string, but this will not be the final representation. It will be changed soon to be a restricted type of tree structure which is currently under development, but the specific nature of that tree structure will not be set in stone until more details of the merge algorithm have been worked out.