Remove option `--dashed`

Remove option `--dashed` of script `plot_eddy_contours.py`. This
option was added in commit 5e0e0deb. The dummy argument dashed of
function snapshot is useful because we can call snapshot several times
on the same axes from elsewhere, but I do not see how the option
`--dashed` of script `plot_eddy_contours.py` is useful, since we
create a new figure when we run the script.

Inst_eddies/Analysis/plot_eddy_contours.py

@@ -144,8 +144,6 @@ if __name__ == "__main__":
                                    "urcrnrlat"))
     parser.add_argument("-l", "--light", help = "lighter plot",
                         action = "store_true")
-    parser.add_argument("--dashed", action = "store_true",
-                        help = "dashed linestyle, useful for a second snapshot")
     parser.add_argument("shpc_dir", help = "directory containing the "
                         "collection of shapefiles")
     parser.add_argument("--save", metavar = "format",
@@ -218,7 +216,7 @@ if __name__ == "__main__":
             print(f"{orientation}: No eddy found")
         else:
             snapshot(ax, ishape_list, SHPC, i_slice, orientation = orientation,
-                     dashed = args.dashed, light = args.light)
+                     light = args.light)
 
     ax.set_title(f"d = {d}", y = 1.05)
     ax.gridlines(draw_labels = True)