Determine appropriate interface to git-annex

[mih]

We want to utilize git-annex as much and as best as we can. At the same time, we will unlikely have it provide all essential components in the near (or even any) future. So we need to settle on a design the places possible implementations as close as possible to git-annex. I see three main pieces

• specification (format) for compute instructions https://github.com/psychoinformatics-de/org/issues/297
• utility to register such a specification in a particular dataset; also see https://github.com/psychoinformatics-de/org/issues/299
• implementation of (a) handler(s) to act on a compute specification record