This is an almost-duplicate file of `report_graph.int`: the difference
is that it processes an edgelist with nodes identified by node index,
instead of date and eddy index. We could make `report_graph.py` more
general instead, but that would degrade its clarity. Let us just have this
almost duplicate file, pending a decision on the format of edgelist.

Move functions `node_to_date_eddy` and `date_eddy_to_node` from
`convert_id.py` to `report_graph_int.py`, where they are used.