SIUnit
The value of the unit converted into the
            unit type stored on the unit. Ex. If the object
            has LengthUnit.DecimalFoot, for a Value of 1.0, this
            will return 3.28084
            
The internal value of the unit.
            
Construct an SIUnit object with a value.
            
Implemented in child classes to control how units are converted
            from a string representation to an SI value.