toString method
It may seem strange to only define (+,-) without (*,/)...
but conceptually things get tricky.
2 kg * 4 kg = 8 kg^2.
2 kg / 4 kg = 0.5 (no unit)
These results are computational devices.
They are not "masses." What is the return type?
kg^2 is used within the context of gravitational attraction, but only
as a factor for calculations-- it doesn't have much conceptual meaning.
Multiplication might result in a single unit raised to a power or u1 * u2
Division might result in only a NumericalValue or u1 / u2.
That complexity would make the API difficult to understand;
although rigourously correct.
This is all very interesting and discussed in:
https://en.wikipedia.org/wiki/Unit_of_measurement#Units_as_dimensions
and
https://en.wikipedia.org/wiki/Quantity_calculus
I'm not a physicist and I don't need it, so, punting. :P
Implementation
@override
String toString() => "$magnitude $unit";