I used `int` before, because it was simple, and I thought _surely_ it
would be enough. I mean, who's going to make a unit whose magnitude has
a prime factor bigger than 32 bits?
This brings us to the "Dalton", a unit whose ratio-magnitude numerator
is 16605390666050. The prime factorization of 16605390666050 is
(2 * 5 * 5 * 53 * 6266185157), and this last number is bigger than 2^31
by a factor of 3 or so.
Fortunately, we should have done this from the beginning anyway, because
otherwise there would be numbers we could represent in `ratio` which we
couldn't represent in `Magnitude`, and this should never be the case.
Chip Hogg
240bdd659c