Clarify and standardize the model type and interaction scheme taxonomy

Currently the app interface specification has mixed semantics.
The field “type”: is e.g. used to encode types of algorithms but also interaction schemes, or even concrete implementations. (see e.g. Radiology App models “deepgrow” and “segmentation_spleen”
That is problematic for a robust implementation in host applications.

First ideas:

1. Interaction types should be separated from “type”
2. The interactions field should be able to take multiple values, as models might support several interaction schemes.
3. The values of the interaction field should be an open set, as we don’t know which kind of new interaction paradigms may come. We should only define the encodings for interaction schemes we already know now and have well defined.
4. It should be better clarified what “type” should encode and if there are a kind of taxonomy that can be used.

The issue names no files, tests, or entry points; begin by locating the app interface specification and reviewing how the type and interactions fields are currently defined and used. Done means the model taxonomy is clarified, interaction schemes are separate and can have multiple values, and the specification documents how known and future interaction values are handled.