def start_notify!(service, characteristic)
      char= _find_characteristic(service, characteristic)
      if char.flag?('notify')
        char.notify!
      else
        raise OperationNotSupportedError.new("No notifications available for characteristic #{characteristic}")
      end
    end
def start_notify!(service, characteristic)
      char= _find_characteristic(service, characteristic)
      if char.flag?('notify')
        char.notify!
      else
        raise OperationNotSupportedError.new("No notifications available for characteristic #{characteristic}")
      end
    end
Registers current device for notifications of the given _characteristic_.
 Synonym for 'subscribe' or 'activate'.
 This step is required in order to later receive notifications.
 @param service [String, Symbol]
 @param characteristic [String, Symbol]