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