input: Write a python function to manage a process with start and stop methods.
  assert process_manager('start') == "Process started"
  assert process_manager('stop') == "Process stopped"
  assert process_manager('invalid') == "Invalid command"
output: def process_manager(command):
      if command == 'start':
          return "Process started"
      elif command == 'stop':
          return "Process stopped"
      else:
          return "Invalid command"