fix width & length recalculation

[MalinSpaniol]

we calculated the width and height of the asparagus pieces from pixel to mm - the recalculation factor is still not correct, and needs to be adjusted so that we get the correct "real" width and hight of all pieces

[sophiasw]

I tried that another time but it seems that it's more difficult to find a threshold which holds for the whole image than I anticipated. It might be an option to "learn" the threshold from the preclassified classes Suppe, Dicke, 1A Anna, 1A Bona and 1A Clara. But then we would have to trust that they are more or less consistent with the sorting.

Also: what do you need this for?