add Levels transform

[Frank-III]

The way I currently store caches is to mix functions and vector which is absolutely not optimal, but I am not sure if I should only allow change the levels or order of CategoricalArray or should generalized for Vector as well. (I guess to allow changing levels is fine as we specified the levels to guarantee it is not an infinite column), and for orders when it is set to be true for a float column the categorical function still work but make no sense.

[codecov-commenter]

Codecov Report

Merging #83 (c4f50b3) into master (48b502f) will increase coverage by 0.35%.
The diff coverage is 96.55%.

@@            Coverage Diff             @@
##           master      #83      +/-   ##
==========================================
+ Coverage   92.73%   93.09%   +0.35%     
==========================================
  Files          22       23       +1     
  Lines         537      565      +28     
==========================================
+ Hits          498      526      +28     
  Misses         39       39              

Impacted Files	Coverage Δ
src/transforms.jl	97.82% <ø> (ø)
src/transforms/levels.jl	96.55% <96.55%> (ø)
src/transforms/parallel.jl	91.66% <0.00%> (-0.14%)	⬇️
src/transforms/select.jl	93.15% <0.00%> (+1.36%)	⬆️

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[juliohm]

This is an awesome PR @Frank-III , thank you for the addition. Left a couple of review comments.

[juliohm]

@Frank-III that trick with the _categorify function was super neat. You avoided a complicated logic by using the first argument of the function as either a function or a vector of levels 👏🏽

Now we just need to add an entry for the new transform in the docs, right after the entry for Functional.

[Frank-III]

Thanks @juliohm, that means a lot to me. It is my first PR and I made a lot of mistakes, I would be more careful with white-space, naming convention and make my code more concise and professional next time. Looking forward to my next PR. :)

[juliohm]

Thank you again @Frank-III for this great addition to the toolkit :) I will go ahead and merge this following our discussions above. We do not want to support mixed Symbol/String types in the same function call.