[FIX] product: pricelist: multiple corrections in UoM-related conversions

Pricelist computations need to consider 2 different Units
of Measure:
 - The default product UoM (product.uom_id), used as reference
   for the various quantities and amounts specified in each
   pricelist rules.
 - The `context UoM` is the UoM in which the result is requested,
   that is the list price UoM.

For example the 'price_min_margin' amount is meant for the unit
price of 1 x default UoM. When the context UoM is not the default
product UoM, it can be any UoM of the same UoM Category, and the
various quantities and amounts specified on the rule need to
be adapted accordingly:
  - min_quantity (expressed in terms of the default UoM)
  - price_surcharge (specified for 1 x default UoM)
  - price_min_margin (specified for 1 x default UoM)
  - price_max_margin (specified for 1 x default UoM)

The UoM corrections were not done consistently and resulted in
wrong prices when computing the price using a non-default UoM.
The cases were a conversion was needed or not were not properly
identified within the _price_rule_get_multi().

After this commit, the various code branches in _price_rule_get_multi
always ensures that:
 - price requested for: `qty` of `qty_uom_id`
 - `qty_in_product_uom` is the requested `qty` converted to default UoM
 - current (intermediary) price: `price` for  `price_uom_id`

Therefore `price` and `price_uom_id` are always in sync, and `price_uom_id`
can always be compared with `qty_uom_id' in order to know whether
a conversion is still needed.

This patch also corrects and extends the regression tests
introduced at revision 79ebe10.