Unit defines a unit type, calculate and compare. The following constructors are supported: Unit() -- empty constructor Unit(i1,i2,i3,i4,i5,i6,i7,i8) -- unit signature Unit(Quantity) -- copy unit from Quantity Unit(Unit) -- copy constructor Unit(string) -- parse the string for units Unit holds the unit type as a string, e.g. 'Area'. Returns the signature.