[IMP] uom: add constraints to force reference factor
[IMP] uom: add constraints to force reference factor

When importing big amount of data, user can import
a list of uom. There is no constraint in Odoo to
force the factor to be equal to 1.0 for the reference
UoM in a category.
Onchange is not enought to ensure that constraint,
so this commit adds one in SQL for data intergrity
and to avoid problem in '_compute_quantity'
method.

ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo