[NOTICE] Plan to deprecate and remove support for the unitPrefix attribute

[VisLab]

The unixPrefix is an attribute of Unit indicating that the unit appears before the value. There is only one unit that has this attribute. It is the $ which has a unit class of currencyUnits. No tag in any existing schema has a unit class of currencyUnits so no existing tag can use this unit.

Proposal for 8.4.0:

1. Create a new unit class called deprecatedUnits. Move $ under this from currencyUnits.
2. Do not allow any future schemas to use any deprecatedUnits.
3. Currently validators will treat using $5 as an error anyway because there won't be a valid unit class.
4. Future validators will flag the value as invalid if the value is expected to be numeric.
5. Users can still use 5 dollars as a value for items that have unit class currencyUnits.

This is a request for comments on this proposal. We will need to add some additional data value tags to allow currency.