option to not shutdown ray if already initialised

[jafermarq]

This a short PR enabling the user to decide whether start_simulation should do ray.shutdown() if an existing Ray runtime is already initialized. In some settings, we might want to have Ray already running before calling start_simulation if, for example, some other tasks have to run first before FL.

[danieljanes]

Good idea, thanks for the PR @jafermarq ! I've approved it, there just a lint issue (too-many-locals) that needs to be resolved. In this particular case I'd say it's ok to just ignore it.