Fixed bug in alpha handling in arc links

[MCPahl]

Thanks for developing and maintaining this tool! I noticed a bug for handling alpha transparency to Matplotlib for link plots.

[lldelisle]

Hi @MCPahl ,
Thank you so much for finding and fixing the bug and providing a high quality test. I changed the target to develop, I merged the develop commits and I added a new plot in the test folder with a changed alpha. I also changed the default value of alpha to 1 to keep the same behavior as before the bug fix.

[lldelisle]

@bgruening would you mind to review this one also?
@MCPahl noticed that the links track as a parameter alpha that was not used. In the help it was written that the default was 0.8 while the plotted alpha was 1. @MCPahl fixed the bug and I changed the default value from 0.8 to 1 to fit with the passed behaviour.

[bgruening]

I always found that file a bit scary :)

But thanks for adding your case here as well.

[lldelisle]

This is more a working file for me to be able to find back how to test locally and keep trace of how I generated the figures...