25 implement duration as a subtype of number

[MicheleCeresoli]

@andreapasquale94 let's discuss whether these could be usefull changes:

1. Duration has been made a chlild of Number. The constructors have been reworked to avoid code repetitions. The constructor Duration(sec::Int, frac::T) has been removed since it equals the default type constructor.
2. Base.convert and Base.promote_rule have been implemented for Duration. Currently Base.convert can be used in two ways (although the first option could be removed):

julia> d = Duration(1)
Duration{Int64}(1, 0)

julia> convert(Float64, d)
Duration{Float64}(1, 0.0)

julia> convert(Duration{Float64}, d)
Duration{Float64}(1, 0.0)

3. The ftype function has been added to retrieve the type in which the fractional part of the Duration is represented.

4. Epoch constructors have also been updated to reflect the above changes and avoid code repetitions.

5. Addition and subtraction implementations of Epoch have been simplified. Additionally, they now properly propagate types. In the previous implementation, summing an epoch of type Epoch{TDB, Float64} with a duration or number of type BigFloat would improperly return an epoch in Float64 representation, leading to a loss in accuracy. Now, the result is expressed using the highest accuracy type (i.e., the one resulting from the promote_rule operation).

6. The step length operation to generate a range of epochs with a daily step range has been re-implemented. Thus ep1:ep2 now properly works again.

Note that even though Duration is a subtype of Number, all operations that have not been defined, i.e., multiplication and division, will return an error upon execution.

[andreapasquale94]

Apart from those minor comments, I would be happy to merge 👯