Multi-cast interactions use case

Attached is a draft version of the use case I was actioned
to do at the last F2F.

Given concrete developments through Burdett ML etc on lower
level operation of choreography, I've considered a slighly
more complex use case at a higher level in order to explore
the boundaries between orchestration (sorry Martin!) and
choreography. And I did so without being up to speed with the
latest conceptualisations of choreography.

So I imposed my own intuitions and thought I'd send out a draft
to get some feedback before doing anything further.

Cheers, Alistair.