Stuttgart: add generic DatexScraperMixin and Stuttgart

[hbruch]

Stuttgart publishes static and dynamic DATEXII publications via MDM (see also this discussion offenesdresden/ParkAPI#36) . As requesting these requires a registration and MDM-provided certificate, I republish (mirror) them under https://data.mfdz.de/. The data itself is published unter dl-de/by-2-0 by the city of Stuttgart.

This PR adds a scraper and geojson for Stuttgart. This scraper uses a DatexScraperMixin which is a slightly generalized version of the current Frankfurt scraper.

Note: the static publication currently doesn't publish some important information like coordinates, type, address. I contacted the data owner and asked for additional information in the publication.

In case this information will not be provided soon, the geojson will need to be enhanced manually :-/

[hbruch]

This PR is ready for review now.

Still missing in the data: information that could be used to determine lot_type.
All parkings are (wrongly) assigned <parkingFacilityLayout>singleLevel</parkingFacilityLayout> in the Stuttgart feed.

Besides the currently extracted information, some more could be extracted as v3 properties:

• opening hours: currently wrongly modelled as start/end period. All parkings, according to the data, are available 7/24 (described as a per weekday period from 00:00:00 to 23:59:00).
• height restrictions
• assigned Parking Spaces, though Stuttgarts seems to used them currently only to distinguish between longTerm and shortTerm parkings. No other assignment class seems to have values > 0 currently.

Such possible extensions are left for future addition.