Fix union and union! of intervals

[iamed2]

For the purposes of these definitions, a "set" of intervals is either a single AbstractInterval or a vector of AbstractIntervals.

This now supports the methods that set union supports. Notably, the method @morris25 found will now throw an ArgumentError rather than falling back to the Base definition and giving a different result.

Fixes #73

Before:

julia> target
10-element Array{AnchoredInterval{-1 hour,ZonedDateTime},1}:
 AnchoredInterval{-1 hour,ZonedDateTime}(ZonedDateTime(2013, 2, 12, 7, tz"UTC"), Inclusivity(false, true))
 AnchoredInterval{-1 hour,ZonedDateTime}(ZonedDateTime(2013, 2, 12, 7, tz"UTC"), Inclusivity(false, true))
 AnchoredInterval{-1 hour,ZonedDateTime}(ZonedDateTime(2013, 2, 13, 7, tz"UTC"), Inclusivity(false, true))
 AnchoredInterval{-1 hour,ZonedDateTime}(ZonedDateTime(2013, 2, 13, 7, tz"UTC"), Inclusivity(false, true))
 AnchoredInterval{-1 hour,ZonedDateTime}(ZonedDateTime(2013, 2, 14, 7, tz"UTC"), Inclusivity(false, true))
 AnchoredInterval{-1 hour,ZonedDateTime}(ZonedDateTime(2013, 2, 14, 7, tz"UTC"), Inclusivity(false, true))
 AnchoredInterval{-1 hour,ZonedDateTime}(ZonedDateTime(2013, 2, 12, 8, tz"UTC"), Inclusivity(false, true))
 AnchoredInterval{-1 hour,ZonedDateTime}(ZonedDateTime(2013, 2, 12, 8, tz"UTC"), Inclusivity(false, true))
 AnchoredInterval{-1 hour,ZonedDateTime}(ZonedDateTime(2013, 2, 13, 8, tz"UTC"), Inclusivity(false, true))
 AnchoredInterval{-1 hour,ZonedDateTime}(ZonedDateTime(2013, 2, 13, 8, tz"UTC"), Inclusivity(false, true))

julia> a = union(target)
3-element Array{AbstractInterval,1}:
 Interval{ZonedDateTime}(ZonedDateTime(2013, 2, 12, 6, tz"UTC"), ZonedDateTime(2013, 2, 12, 8, tz"UTC"), Inclusivity(false, true))
 Interval{ZonedDateTime}(ZonedDateTime(2013, 2, 13, 6, tz"UTC"), ZonedDateTime(2013, 2, 13, 8, tz"UTC"), Inclusivity(false, true))
 Interval{ZonedDateTime}(ZonedDateTime(2013, 2, 14, 6, tz"UTC"), ZonedDateTime(2013, 2, 14, 7, tz"UTC"), Inclusivity(false, true))

julia> b = union!(target)
5-element Array{AnchoredInterval{-1 hour,ZonedDateTime},1}:
 AnchoredInterval{-1 hour,ZonedDateTime}(ZonedDateTime(2013, 2, 12, 7, tz"UTC"), Inclusivity(false, true))
 AnchoredInterval{-1 hour,ZonedDateTime}(ZonedDateTime(2013, 2, 13, 7, tz"UTC"), Inclusivity(false, true))
 AnchoredInterval{-1 hour,ZonedDateTime}(ZonedDateTime(2013, 2, 14, 7, tz"UTC"), Inclusivity(false, true))
 AnchoredInterval{-1 hour,ZonedDateTime}(ZonedDateTime(2013, 2, 12, 8, tz"UTC"), Inclusivity(false, true))
 AnchoredInterval{-1 hour,ZonedDateTime}(ZonedDateTime(2013, 2, 13, 8, tz"UTC"), Inclusivity(false, true))

After:

julia> a = union(target)
3-element Array{AbstractInterval,1}:
 Interval{ZonedDateTime}(ZonedDateTime(2013, 2, 12, 6, tz"UTC"), ZonedDateTime(2013, 2, 12, 8, tz"UTC"), Inclusivity(false, true))
 Interval{ZonedDateTime}(ZonedDateTime(2013, 2, 13, 6, tz"UTC"), ZonedDateTime(2013, 2, 13, 8, tz"UTC"), Inclusivity(false, true))
 Interval{ZonedDateTime}(ZonedDateTime(2013, 2, 14, 6, tz"UTC"), ZonedDateTime(2013, 2, 14, 7, tz"UTC"), Inclusivity(false, true))

julia> b = union!(target)
ERROR: ArgumentError: Cannot `union!` array of intervals of type AnchoredInterval{-1 hour,ZonedDateTime} as the type may change
Stacktrace:
 [1] union!(::Array{AnchoredInterval{-1 hour,ZonedDateTime},1}) at /Users/ericdavies/.julia/dev/Intervals/src/interval.jl:295
 [2] top-level scope at none:0

[omus]

It would be better to use dispatch here for future interval types can support their own union! methods

function Base.union!(intervals::AbstractVector{<:AbstractInterval})
    throw(ArgumentError("Cannot `union!` array of intervals of type $(eltype(intervals)) as the type may change"))
end

Base.union!(intervals::AbstractVector{<:Union{AbstractInterval,Interval}) = ...

(I think this works)

[iamed2]

Doesn't the existing definition already allow that?

[omus]

I think my point here was that this approach in the PR isn't the best for new interval types that want to define their own union! method.

[omus]

Fixes: #73

[omus]

Should we do:

Suggested change

	interval::Union{AbstractInterval, AbstractVector{<:AbstractInterval}}, intervals...,
	interval::Union{AbstractInterval, AbstractVector{<:AbstractInterval}}, intervals::Union{AbstractInterval, AbstractVector{<:AbstractInterval}}...,

Avoids a call such as union(Interval(1,2), 3) causing a confusing error