Custom timescales are not supported by Epoch constructors

[MicheleCeresoli]

When a custom timescale is defined, not all the epoch constructors support it:

julia> using Tempo
julia> @timescale ET EphemerisTime

julia> ep = Epoch(12, ET)
2000-01-01T12:00:12.0000 ET

julia> ep = Epoch("12 ET")
ERROR: UndefVarError: `ET` not defined

This is because the constructors look for the timescales that exist within the scope of the package and not those that have been defined outside of it.

[andreapasquale94]

This is for the eval we do have in the Epoch(str::AbstractString) constructor. Investigating how to solve it?