Use of non-public API in extracting levels

[ablaom]

In the example below we have x.pool.levels which I don't believe is public API. You can instead do CategoricalArrays.levels(x) for a CategoricalValue or a CategoricalArray. After CategoricalArrays.jl v1.0, this returns a CategoricalVector (previously it returned the unwrap of that). I am separately posting a PR to force v1.0, currently you have 0.10. After that update, you may possibly need to apply CategoricalArrays.unwrap to the result of levels(x) to get "raw" values, if you really need them.

https://github.com/cjdoris/ARFFFiles.jl/blob/a93f9d5c4596e9ccce6b7be2b80e5d960ce6c39b/src/ARFFFiles.jl#L1173

There are other examples of the improper use. I found three with this search: https://github.com/search?q=repo%3Acjdoris%2FARFFFiles.jl%20levels&type=code

cc nalimilan

[cjdoris]

Fixed in #33, thanks.