Users can configure multiple references in a same
category. This might cause problems during conversion
and does not make sense.
Since this is a "late" constraint, we define it in
python to avoid conflict with existing data.
We allow user to have multiple unactive reference, but
force to have exactly one active. So when a new UoM
category is created, user have to encode the reference
unit first, and then the bigger/smaller ones.
This commit also update stock tests, since they were
redefining new reference UoM in the same category, and
finally, this commit provides some tests for this
uniqueness and custom UoM behavior.
Task #1820076
jem-odoo
e0ed50a724