#189 Participant types and the incremental method

As part of the "SRT incremental type annotation method" (SITAM), please allow us to associate a scenario to a Choreography without requiring the Participant.Type to be defined.

Using SITAM, first I identify my Participant instances: gary, matthew, steve. Only later do I identify and annotate with their Participant types: gary:Committer, matthew:Tester, steve:Methdologist.

There is a parallel here with UML which distinguishes clearly between two concepts to allow incremental development:
* Valid UML - is the UML document a valid UML model.
* Complete UML - is the UML model complete.

My perception is that SITAM is about distinguishing between Valid and Complete choreographies and scenarios. A scenario may be valid withou being complete.