expand the "save path" properly

inspired by: https://stackoverflow.com/questions/51520/how-to-get-an-absolute-file-path-in-python

src/main.py

@@ -219,7 +219,7 @@ if __name__ == "__main__":
         success()
 
     if args.save is not None:
-        filename = os.path.expanduser(args.save)
+        filename = os.path.abspath(os.path.expanduser(args.save))
         fig.set_size_inches(args.fig_size, forward=False)
         plt.savefig(filename, dpi=args.dpi)