GitLab

This includes:
    - Interfaces for edge and intersection based relevance
    - A helper structure providing the required information
          for relevance calculation

- now supporting Attraction Points via core API
- allowing location accuracy and modifications
- SiS-Error model interface

Pub/Sub unsubscribe() on listener

Full support for multiple listeners on the same subscription

See merge request !27

Br/a1 master merge

(As discussed during the MAKI BarCamp 2016)

See merge request !26

GraphElementProperties