Add GeoJSON of tram stations for E-Tram and Cargotram

Add /api/tram_stop route for the provided GeoJSON
Refactoring of loadData to reduce duplicate code
Rename station CSV as it is independent of the date
Rename 'tram_stop' to station to match special waste data
Add data for year 2015 and load it
Add `limit` parameter to reduce output of API if needed
Update CHANGELOG