TAI-to-UTC transformation prior to 1972

[MicheleCeresoli]

The UTC timescale is officially defined starting on 1 January 1960. As discussed in skyfielders/python-skyfield#679, prior to the introduction of leapseconds on 1 January 1972, the relationship between TAI and UTC is governed by linear offsets, which are reported in:

https://hpiers.obspm.fr/eop-pc/index.php?index=TAI-UTC_tab&lang=en

At the moment, if someone attempts to convert from TAI to UTC or viceversa prior to 1972 the following error is thrown:

julia> ep = Epoch("1961-01-01T12:00:00 UTC")
1961-01-01T12:00:00.0000 UTC

julia> convert(TAI, ep)
ERROR: EpochConversionError: cannot convert Epoch from the timescale CoordinatedUniversalTime to InternationalAtomicTime.

Stacktrace:
 [1] convert(to::InternationalAtomicTime, e::Epoch{CoordinatedUniversalTime, Float64}; system::TimeSystem{Float64})
   @ Tempo ~/.julia/packages/Tempo/Ba1ZM/src/epoch.jl:233
 [2] convert(to::InternationalAtomicTime, e::Epoch{CoordinatedUniversalTime, Float64})
   @ Tempo ~/.julia/packages/Tempo/Ba1ZM/src/epoch.jl:225
 [3] top-level scope
   @ REPL[9]:1

caused by: BoundsError: attempt to access 28-element Vector{Float64} at index [0]

The proposal is to implement within the tai2utc conversion, the linear offsets from 1961 to 1972. On the other hand, when attempting the UTC-to-TAI conversion prior to 1961, either a null offset (with a warning) or an error should be returned.

[andreapasquale94]

Agreed not to do that.