Add chord-length type particle size method

[fussegli]

Yes, that's the old simple method when people were measuring line per line the particle size.
Yet, I think it is worth adding to the toolbox as it's a method that add interesting information on anisotropy. Besides, I am really curious to see how such simple, fast method compare with the other much more complex and CPU expensive methods (watershed and PCRF, i am looking at you!)

I set "help wanted" because this item is at the bottom of my priority list, so if you want to collaborate to this project (and be credited for your effort of course) please let me know.

[fussegli]

Finished. Will be released in the next version of the characterization module