Clean messy outlines

Based on Robofab’s ThresholdPen, the scripts takes care of four tasks:
– Saving anchors (which would otherwise be destroyed by the
ThresholdPen)
– Moving aligned points (i.e if there’s a useless point on a straight
line)
– Redrawing the glyph through the Threshold point to get rid of
unnecessary points (default threshold value is of 4 units)
– Replacing the anchors at their initial position