Returns the list of timesteps when survey measures has been captured
        None if xml document is mailformed
        xpath: /scenario/monitoring/survey
        https://github.com/SwissTPH/openmalaria/wiki/GeneratedSchema32Doc#survey-times-time-steps

def surveys(self):
        """
        Returns the list of timesteps when survey measures has been captured
        None if xml document is mailformed
        xpath: /scenario/monitoring/survey
        https://github.com/SwissTPH/openmalaria/wiki/GeneratedSchema32Doc#survey-times-time-steps
        """
        survey_time_list = list()
        # Extract surveyTimes from /scenario/monitoring/surveys section
        # Using root element instead of xpath to avoid problems with namespaces
        # (root tag was <scenario> prior to schema 32, and then it was switched to <om:scenario>)
        try:
            for item in self.et.find("surveys").findall("surveyTime"):
                # Converting to float first to allow values like 730.0
                survey_time_list.append(int(item.text))
        except AttributeError:
            return None
        return survey_time_list