Implement stop simulation action

Method to stop subprocess in local simulation pool already exists, it will kill the subprocess.

**Todos:**

* Have to connect with frontend action
* Handling of stopped simulation -> what state should should be set? ERROR? Or introduce new state "STOPPED"/"CANCELED"? Do we clean up any created files during simulation?

Start by locating the existing local simulation pool method that kills the subprocess and the frontend action flow that controls simulations. Define how a stopped simulation should be represented, including whether to add a STOPPED or CANCELED state, and decide whether simulation-created files require cleanup; done means the frontend action stops the subprocess and consistently handles the resulting state.

Written by the indexing model from the issue text.