Support pulling models, and manage ollama server by PID

Main changes:

• can now pull models
• can see info about existing models, ollama client version, currently set model, etc
• StartServer now checks if an ollama process is running yet, and if not, starts the ollama serve process.
  • in general, we no longer directly manage a exec.Cmd instance. Instead, we return the PID.
  • StopServer was removed; this can now be done by killing the ollama process PID, if you want.