Write node index instead of date and eddy index to the edgelist
file. Motivations: stay closer to Matlab, at least for now; some graph
processing programs other than NetworkX will not easily read nodes
that are tuples. Consequences of this change are that we no longer
need `date_num` from the Matlab files and we no longer need the
functions `date_index` and `convert_id` in the script
`overlap_v6.py`. So move these functions for now to a new file
`convert_id.py`.