Define attribute `coordinates` of nodes instead of separated
dictionary pos. Use function `report_graph.read_eddy_graph` to define
the attributes of nodes. So we do not need the functions
`compute_ishape` and `get_coord` any longer. Note that, for visible
eddies, `plot_traj.py` used to loop on nodes of the graph and access
corresponding shapes, while `report_graph.read_eddy_graph` loops on
the shapes and accesses corresponding nodes. This is probably quicker.