ENH: Adding Units Support for Definitions and Attributes

You can now assign units to Definitions and Attributes. This can be useful when an Attribute represents a concept that has units but does not have an explicit value. For example, if an Attribute represents a temperature field that would be created by a simulation, you may want to assign a default unit to its Definition (such as Kelvin) but allow the user to change the Attribute's units to Celsius.

The rules for assigning local units to an Attribute that override those inherited through its definition are the same as the case of assigning a value with units to a ValueItem.