Implement Interaction model

[brianjmiller]

Need to implement the various interaction parts of the Statement model.

[tomiblack]

Are you planning to implement it in the near future?

[brianjmiller]

I don't have any specific plans to which is to say it is not on our roadmap. Pull requests are great! Can also consider bounty based feature additions. I don't recall if there was a particular difficulty with them or if it was just a lack of time. The Activity Definition class already has them stubbed out in the comments.

[brianjmiller]

See #17.