add thread class (stitch one, not computing) to deal with color, width and later (maybe) rendering style

- bug fix in curves (the subcurve function return bad path in case of start and end both on the same segment)

- correction d'un bug lorsqu'une subcurve etait créee avec startPoint=endPoint (les curves cycliques ne fonctionnaient pas dans tous les cas)

add a maxlen option for the birail stitch (add intermediate point when a stitch is longer than maxlen)

refresh doxygen documentation