[FIX] let users correct stock when moves were done without prodlots

Fixes #1651: When a location contains product X with lot P, in v7 users can make stock moves without a lot: then the source location will have :
    +quantity with lot P
    -quantity without lot.
Users will often want to make an inventory to transform this into :
    0 with lot P
    0 without lot. But this is not possible because the current code considers this is an 'incoming move' and forbids it. This is wrong : inventory moves must not be considered incoming or outgoing, they should always be allowed in order to let users match their computer inventory with the real world.