code: def changeHS(self):
          
          health_systems = []
          change_hs = self.et.find("changeHS")
          if change_hs is None:
              return health_systems
          for health_system in change_hs.findall("timedDeployment"):
              health_systems.append([int(health_system.attrib("time")), HealthSystem(self.et)])
          return health_systems
docs: Change health system interventions
          https://github.com/SwissTPH/openmalaria/wiki/GeneratedSchema32Doc#change-health-system
          Returns: list of HealthSystems together with timestep when they are applied