Division for negative number was rounding up

[dahlend]

This is normal behavior for negative numbers on CPUs, but unexpected behavior for math. This required changing the divisions to div_euclid to match the expected behavior.

     let x = -5 / 3;
     assert!(x, -1);

     let y = -5.div_euclid(3); // this is the mathematically expected behavior
     assert!(y, -2);

Fixes #70

Here is sweeping JD back to -1e6 JD, and performing a YMD conversion for each day:

This is now expected behavior.