GPS epoch

[bhawkins]

This package helpfully defines GPS_EPOCH here https://github.com/JuliaAstro/AstroTime.jl/blob/a891ffef6fb142d37b8f8836b3ec75da4955606d/src/Epochs/epoch_constants.jl#L7

However, I think it's incorrect to define it in TT. It should be 1980-01-06 UTC according to wikipedia and also section 11.4.3.3 of the manual from the US Coast Guard. So the definition should be equivalent to from_utc(1980,1,6) or TAIEpoch(1980, 1, 6, 0, 0, 19.0).