functions of 'timeseries.py' have error handling
Some functions within 'toardb/timeseries/timeseries.py' abort uncontrollably if the function is called with wrong values. Examples for this are 'get_citation' or 'get_timeseries_changelog' these functions can get 'None'-values back by calling functions with the same name from 'crud.py', which are not caught within 'timeseries.py'.