Make flower-simulation accept ClientApp and ServerApp objects

[jafermarq]

Enables running the new Simulation Engine on Notebooks. For this to be possible we need:

• to update run_simulation so it accepts ClientApp and ServerApp objects and not just strings pointing to modules where those are defined.
• Wrap the whole execution into a new thread (which will itself launch another one running the ServerApp). This is needed since notebooks have their own Asyncio event loop running, conflicting with some parts of the Simulation Engine if they run on the main thread. Running on the main thread remains the default behaviour when the code is executed outside a notebook environment (whether it is from the cli via flower-simulation or from a python script)

• Decide if we apply the changes to the notebooks (or keep it in a separate code cell?) -> in a future PR.