[FIX] product: precision of uom

Remove the hardcoded precision of 12 on factor and factor_inv to avoid rounding errors.

e.g. a unit with a factor_inv of 6.0 used to be computed as:
factor_inv: 6.0 -> factor: 0.166666666667 (1.0/6.0, rounded to 12 digits) -> factor_inv: 5.999999999988 (1.0/factor)
which could lead to errors such 12*0.166666666667 = 2.000000000004 instead of 2.0

Added the option display_digits on float fields in views, to overwrite default value of [69,2].
[FIX] product: view of product.uom

Add readonly attribute to avoid sending both factor and factor_inv value.
This was possible if the user switches between uom_type to fill the two fields.