Add digits mixin
A common pattern is to create Function
fields to store the digits to use to display a number. Usually the digits come from a referential record like the currency or the product unit.
To simplify the declaration I propose to allow to use a field name as digits value. In such case the field should point to a Model with a get_digits
method (which will be cached) that returns the real tuple to use.
The ModelStorage
should be adapted to support such definition but also the clients which should make RPC calls (like for the symbol).