Added a gross tally/ net tally administration functionality

When buying and selling hardwood the terms gross tally (or sometimes called green tally) and net tally are important to know.

On an item it is now possible to set up a Gross tally/ net tally % which can be used on selling, to influence the quantity calculation and with that to decide if the timber is sold wet (green tally) or dried (net tally).

For further information have a look into the timber docs documentation: http://docs.acadon.de/products/timber/en-US/timber/grosstallynettallyhandling.html