Add serial number functionalities to lot

Following the discussion on https://discuss.tryton.org/t/serial-numbers-on-products-and-stock-moves/137 we can implement serial numbers as lot numbers.

assigned to @nicoe

added type::feature + 1 deleted label

review28251002 updated at https://codereview.tryton.org/28251002/#ps1

review28251002 updated at https://codereview.tryton.org/28251002/#ps20001

review28251002 updated at https://codereview.tryton.org/28251002/#ps40001

review28251002 updated at https://codereview.tryton.org/28251002/#ps60001

review28251002 updated at https://codereview.tryton.org/28251002/#ps80001

review28251002 updated at https://codereview.tryton.org/28251002/#ps100001

review28251002 updated at https://codereview.tryton.org/28251002/#ps120001

review28251002 updated at https://codereview.tryton.org/28251002/#ps140001

review28251002 updated at https://codereview.tryton.org/28251002/#ps160001

I'm wondering if we should also check that the quantity of an inventory is not greater than 1 for serializable products.

review28251002 updated at https://codereview.tryton.org/28251002/#ps180001

review28251002 updated at https://codereview.tryton.org/28251002/#ps200001

I think the current proposal is too strict by defining the serial property only on the product.
Indeed I think we could have serial number for a pack of products. This will be expressed by a different unit of measure with still 1 as quantity.
The same product could have different kind of serial number, one when it was in a bigger packaging and one when it is unpacked.
For this, I think that the serial property should be stored on the lot itself.
This serial property will be stored as a UoM which represent the total quantity that contains the lot. This way the constraints will be that a move with this lot can not have a quantity greater than the 1 UoM of the lot (and for shipment constraint the sum can not be greater).
The serial case will be managed by using the smallest UoM with 0 digits and rounding of 1.

As serial number will be just a particular case of this module, I think we should have a better name. I propose stock_lot_unit

assigned to @ced and unassigned @nicoe

added 1 deleted label and removed 1 deleted label

I updated the review to implement design of msg41322
The behavior did not change too much as the scenario had received only small changes.

added 1 deleted label and removed 1 deleted label

review28251002 updated at https://codereview.tryton.org/28251002/#ps220001