An interface for classes that can participate in numeric operations. All the numeric operation (+, -, *, ...) are expressed in terms of this Numeric interface. Numerics has a value (getNumericValue) and a dimension (getDimension). Numerics can be either absolute or relative. Relative numerics must be resolved against base value before the value can be used.
To support relative numerics internally in the expresion parser and during evaulation one additional methods exists: isAbsolute() which return true for absolute numerics and false for relative numerics.