Document convinient unit definitions

[ali-ramadhan]

We should document the following constant definitions in utils.jl, which I think are useful and make scripts more readable, but would be quite confusing if you didn't know that they are just Float64 constants.

const second = 1.0
const minute = 60.0
const hour   = 60minute
const day    = 24hour

KiB, MiB, GiB, TiB = 1024.0 .^ (1:4)

I'd also like to add constants for meter, centimeter (cm?), and kilometer (km?).