Move function `partition_graph_date` from script `report_graph.py` to
script `edgelist_to_Graphviz.py`. `partition_graph_date` is no longer
used in `report_graph.py`.

Move functions `node_to_date_eddy` and `date_eddy_to_node` from script
`report_graph.py` to script `util_eddies.py`. `util_eddies.py`
contains more basic functions, it only imports
shapefile. `report_graph.py` imports networkx. It was not convenient
nor meaningful to require networkx for scripts that use
`node_to_date_eddy` or `date_eddy_to_node`.

This can happen if the input graph file contains only an extracted
component.

Instead of reading it from standard input.

This carries more information.

So we do not have to symlink the file `traj_segm.json`.

Because we want to add an option.

Accept input graph of segments without cost functions and accept
absence of file `traj_segm.json`.