Update functions in services

[Kebslock]

Currently there are several functions in each service updating the related models e.g. setting start and end point of a track and updating the GeoJSON-path (one function for each). This is not a good design as the database has just one function to update for each model and also the frontend sends all updated data at once. If there is any business logic to do, the API / routes handling needs to handle each sent value from the frontend independently and call one service function for each.

So all update functions on service level should be aggregated to one function per model. A workaround is to do the business logic in the API (see #89 for example).

[Kebslock]

This issue will be resolved by #113.