Zero-based graph_id numbering

The current graph_id-numbering starts from 1, but e.g. the simulation output starts from 0. This should be consistent (and starting from 0). May require a bit of modification to tests etc.

changed milestone to %Autumn version

added Code quality label

marked this issue as related to #97 (closed)

created branch 255-zero-based-graph_id-numbering to address this issue

mentioned in merge request !417 (merged)

closed with merge request !417 (merged)