Add check if start-date is smaller then or equal to end-date

[Andrerg01]

Is your feature request related to a problem? Please describe.
Put bad dates on config and took me a long time to figure out why query was coming back empty

Describe the solution you'd like
A simple line on querying functions to check for date compatibility at /src/backend/opensky_query.py, and raise an error if start comes after end
Having a line like that on scripts/gather_data.py would be good

Describe alternatives you've considered
Not be dumb like me

Additional context