New Functional

[eliascarv]

This PR redesigns Functional implementation to add support for applying functions to specific columns.
The following constructor has been added:

Functional(col₁ => func₁, col₂ => func₂, ..., colₙ => funcₙ)

closes #59

[juliohm]

@eliascarv is this PR really ready for review? I got the notification but it seems like you are still working on it? There are many places still mentioning the single function signature.

[eliascarv]

You're right, I'll switch to WIP.

[codecov-commenter]

Codecov Report

Merging #60 (1c7cae1) into master (c50d658) will decrease coverage by 3.16%.
The diff coverage is 35.71%.

@@            Coverage Diff             @@
##           master      #60      +/-   ##
==========================================
- Coverage   92.84%   89.67%   -3.17%     
==========================================
  Files          19       19              
  Lines         461      494      +33     
==========================================
+ Hits          428      443      +15     
- Misses         33       51      +18     

Impacted Files	Coverage Δ
src/transforms/functional.jl	61.22% <35.71%> (-32.53%)	⬇️

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[eliascarv]

This PR is now ready for review @juliohm

[juliohm]

Awesome 💯