stretch algorithm (second try)

This is a patch I used to fix incorrect diameters of vertical holes and, in general, insufficient inner dimensions. I created a new parameter "stretchDistance". When this parameter is zero (the default), nothing new occurs. When the parmeter is 600 - for my printer - the system increases diameters in a manner all diameters are accurate, even small ones.

The idea behind the algorithm is that the command sent to the printer is like the position of the front wheel of a bike, and the real position of the plastic is the position of the rear wheel. stretchDistance is the distance, in microns, between the front and the rear wheel.  Maths : http://www.mathcurve.com/courbes2d/tractrice/tractoiredecercle.shtml