Redesign of AstroPeriod née Period and other small refactorings

[helgee]

Successful "Conference-Driven Development" 🤣

This a redesign of the AstroPeriod (formerly known as Period) type which makes it more compatible with the Epoch data model and also enables it to track its associated floating point error.

struct AstroPeriod{U<:TimeUnit, T}
    unit::U
    second::Int64
    fraction::T
    error::T
end

[giordano]

Looks good as far as I can tell 🙂

[giordano]

I don't know if that's relevant, but isfinite is false also for NaN.

[helgee]

Interesting question 🤔

Here is Orekit's answer about what to do with NaN which is kind of unsatisfactory 🤣:

AbsoluteDate date = new AbsoluteDate(2021, 7, 4, 0, 59, 13.123, TimeScalesFactory.getTAI());
AbsoluteDate newDate = new AbsoluteDate(date, Double.NaN);

// Returns 2021-07-04T00:59:NaN