Add option save

For the test of the script in `tests.json`. Normally, we would save to
pdf, but pdf format includes a date stamp. So pdf is not convenient
for testing. So we include the format value in argument save and we
save to png for the automated test.

Analysis/plot_traj.py

@@ -95,6 +95,8 @@ if __name__ == "__main__":
                         "shapefiles, with visible eddies at all dates")
     parser.add_argument("-l", "--label",
                         choices = ["root", "leaf", "split", "merge"])
+    parser.add_argument("-s", "--save", default = "pdf", metavar = "format",
+                        help = "Save file to specified format")
     group = parser.add_mutually_exclusive_group()
     group.add_argument("-n", "--node", help = "Select component containing "
                        "node, with orientation cyclo or anti", nargs = 3,
@@ -171,4 +173,9 @@ if __name__ == "__main__":
     ax.coastlines()
     ax.gridlines(draw_labels = True)
     print("Elapsed time:", time.perf_counter() - t1, "s")
-    plt.show()
+
+    if args.save:
+        plt.savefig(f"plot_traj.{args.save}")
+        print(f'Created "plot_traj.{args.save}".')
+    else:
+        plt.show()