[FIX] stock: forbid adding new lines when processing picking

Source and destination locations are required and not displayed in the form view.
Adding new items when recieving a picking can not be easily guessed as we can put different locations for each line, using default locations may not be the expected result.
Instead should modify the original picking or create new one.
Fixes #2074, opw 612768
[FIX] sale_margin: cost price uom

When computing the cost price to display, take the uom into account. The cost of 1 Unit or 1 dozen should not be the same. opw 599727