fix categorical for views

[bkamins]

The problem is:

julia> c = categorical([1,2,3], ordered=true)
3-element CategoricalArray{Int64,1,UInt32}:
 1
 2
 3

julia> c
3-element CategoricalArray{Int64,1,UInt32}:
 1
 2
 3

julia> isordered(c)
true

julia> cv = view(c, :)
3-element view(::CategoricalArray{Int64,1,UInt32,Int64,CategoricalValue{Int64,UInt32},Union{}}, :) with eltype CategoricalValue{Int64,UInt32}:
 1
 2
 3

julia> isordered(cv)
true

julia> isordered(categorical(cv))
false

The core of the issue is how _isordered function is implemented (as it is not extensible now unfortunately). Probably a temporary fix is to define _isordered for SubArrays.

[nalimilan]

Did you mean to file this in CategoricalArrays? But yes, I think we can define _isordered to call isordered for any AbstractArray{<:CategoricalValue} which implements that function -- maybe falling back to checking whether each entry is ordered.

[bkamins]

Yes in CategoricalArrays.jl (I am in parallel looking at CategoricalArrays.jl, CSV.jl and DataFrames.jl so I have mixed this up - sorry for this). I am moving the issue there

[nalimilan]

You know we can transfer issues know? :-)