Use decimal fields for the min / max quantity traits

Hm... the fields should have been made decimal fields from the start, changing this now requires an update hook and updating all variations...
We now have a step_order_quantity. For consistency sake, it's also defined as an integer...
If this is ever fixed, the 3 fields would have to be addressed at the same time.
There is a much simpler workaround I suppose... by defining 3 additional traits (maximum_order_quantity_decimal, mimum_order_quantity_decimal and step_order_quantity_decimal).

This isn't an elegant solution but at least removes the need for an update hook... This does make the UX more confusing though...