Production/Box build linking sub-units based on serial AND part number

As it is now, linking of sub-units (or parent units) can only be done by assigning a parentSerialNumber. 

This means that the possibility for using the same serial number in several levels of the build (which is not uncommon) can not be done while employing the functionality of Production. Basic Wats functionality, with linking of sub-units in test reports and such does support this, so it is surprising that the Production module does not. 

We propose that you add the possibility for inputing parentSerialNumber and parentPartNumber, to eliminate this limitation.