Updating stock move internal quantities when changing quantity
The `.write` method ensure that internal quantities are always up to date with the quantity. But to do that it needs to perform a second call to write. It will be more efficient if the internal quantities were written on the first call. Such that `.write` will just check the internal quantities and not find the need to update them. For that I propose to add an instance method which recompute and fill the internal quantities on a stock move instance. This will be used (optionally) when the quantity or the unit of an existing stock move is modified.
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