Take context into accont when searching for prodlots

The current implementation of _stock_search returns prodlots from every location.
This proposal makes it context-sensitive, so that it can be limited to a subset of locations and/or products.
Do you explicitly want that if context = `{'location_id':False}` we do *not* search? If yes, probably worth adding a comment to say why we did not use `context.get('location_id')`