Improvement/updates to multiinstance

[essweine]

This PR adds a task_info method that can be used to provide runtime information about tasks, based on the task spec and the task itself. The immediate requirement is to provide information about multiinstance tasks; however, this is a generic method that can be extended by any spec type to provide additional information to a process engine that is defined by that type based on the current state of the task.

Additionally, it

• adds the ability to choose whether pre and post scripts on MI task specs in the spiff package should be attached to the MI task or the instance
• raises an error if the specified input or output item of an MI task is already present in the task data

[danfunk]

So we have additional work it the backend and frontend to pick up this information and surface it in the UI? And additional work in the BPMN-IO to set the flag on when to execute the pre and post scripts?

[essweine]

I've already done the bpmn.js work to select where the scripts go, I just haven't opened a PR for it because this should be merged first (to avoid having something in the modeler that doesn't do anything).

The reason for adding the task_info method is to make it easier handle the UI in the frontend and is the stuff I've been working on this week. Spiff mostly already maintained this stuff in its internal data, this just exposes it in an easier way than doing db queries on the internal data of the tasks.