Re-attach to running process, or kill process on page close

As it is now, when a background process is started like in the [lorenz demo](https://github.com/GenieFramework/GenieFrameworkDemos_NewAPI/tree/main/SystemSimulation),
the process keeps running when the user closes the window or reloads. We have two choices:

1. Kill the process on window close
2. Keep it running and let the user re-attach to the running process

For 1 we'd need to send a kill signal to the running task. However, this is an issue in Stipple since having a reactive `Task`variable yields serialization errors. @hhaensel suggested we could modify the Genie serializer to fix this

For 2, we'd need to keep track of the user session and manually return the corresponding `ReactiveModel`on the route.