Allow use a BOM just for manf#

[hildogjr]

KiCost could allow use a generated BOM just to group and use the manf# without show at the spreadsheet. This emerge from a @timoalho discussion. It needs #359 and #266 fix first.

As first step, set all quantity to 0, so this issue will have the fix development merged with #358 (need #359 first). After, a additional fix (second step) setting the quantity to None to identify and remove the project reference after fix #266.