Running non-janis workflows with the assistant

[illusional]

We sort of suggest it would be possible in the talk, and I always thought it shouldn't be too technically hard to implement (as we convert the workflow to CWL / WDL anyway).

This PR tests packed CWL workflows with cwltool which WORKS!

There are a number of things that haven't been implemented yet:

• Type of workflow spec (CWL / WDL) - this changes which engines are compatible. The assistant was designed with this, and knows which specs each engine can support.
• Output collection (Janis has previously inferred from the outputs from the task description), we would need to change the way this works (and you'd lose the fancy janis output naming)
• Required a PACKED workflow. We could very theoretically pack it for the user.
• Inputs are expected to be already formatted for the engine, include the class: File annotation for cwltool, or only json formatted for Cromwell.
• CWL support for cromwell is unclear, it would be good to clear up how to submit workflows again to Cromwell.
• + I'm sure many more things.

But this PR shows that Janis can be modular and you could jump out (or IN) at different stages.