[FIX] stock: backorder uom issue
[FIX] stock: backorder uom issue

Use case to reproduce:
- Sell 1.1 unit
- Validate the picking with 1 unit
- With or without backorder the initial demand is 0.999
- The backorder initial demand is 0.101

It happens because _compute_quantites uses 'UP' by
default. Since 1.1 - 1.0 equals 0.10000000000000009
we face the problem.

This commit uses an 'HALF-UP' instead. It should not generate
any problem since float_compare in order to decide if we need
a back order or not use a float_round on the same precision_digits

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