Human-readable time durations

[sharkdp]

This allows us to do things like

>>> 1 sidereal_day // human

  = 23 hours + 56 minutes + 4.090500 seconds    [String]

Or

>>> ten million seconds // human

  = 115 days + 17 hours + 46 minutes + 40 seconds    [String]

Once we implement #202, we can also write those as conversions (1 siderreal_day -> human)

related: #181

[triallax]

I know it'd be a bit more verbose, but it would be nice if the code output e.g. 1 hour instead of 1 hours.