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]

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]

I thought of making _isordered check if isordered is defined for the passed argument and if yes reuse it and return false otherwise.

The problem is that AbstractArray{<:CategoricalValue} can contain values from different categorical arrays (even if they are all ordered they can even have conflicting orders). So if it does not define isordered I would fallback to false.

[nalimilan]

Yes, if isordered is not defined then the result will probably not be very useful. (Though in theory values from different pools could have compatible orders.)

[bkamins]

Though in theory values from different pools could have compatible orders

Yes, but checking this would have a big performance overhead, and as noted - I am not sure if it would be useful.