Issue #116: Add --path-arrows option for arrow rendering

[napakalas]

Issue #116

• Updated the draw_arrow() function to do nothing by default. Introduced a new --path-arrows argument to enable arrow rendering on paths. Updated the README to reflect the usage of this new option.
• Check circular path integrity by verifying centerline and nodes to ensure circularity. This resolves rendering issues with keast-4 and corrects rendering for unbranched-20 and unbranched-21. The following figure illustrates the keast-4 path before and after the update.

keast-4 before update

keast-4 after update

[dbrnz]

OK, I had to "Review changes" and then "Submit review"...