Fix Unwanted Bézier Curves and Improve Path Connections

[napakalas]

#172

• Fixes path rendering issues where Bézier smoothing caused wavy lines and small loops at node connections.
• Adds a --bezier-smoothing option (angle tolerance 0.1)
• Introduces connect_centre() for use by draw_line() to improve path continuity and visual clarity.

[napakalas]

Before and after update:

Rendering on vagus nerve (the busiest nerve):

[dbrnz]

generation_options.add_argument('--bezier-smoothing', dest='bezierSmoothing', action='store_true',
     help="Render bezier paths with angle tolerance set to 0.1")

It's not too clear what the --bezier-smoothing option is about. Where does the 0.1 come from? Looking at the code, the default is to have a tolerance of 0 -- this should be mentioned in the help text.

Which of the above pictures have the option turned on?

What value is intended to be the default when generating maps?