DTableColumn improvements

[krynju]

DTableColumn implementation was pretty rushed, so addressing the debt

[codecov-commenter]

Codecov Report

Merging #12 (de93a0e) into main (826abbe) will increase coverage by 2.16%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #12      +/-   ##
==========================================
+ Coverage   88.29%   90.45%   +2.16%     
==========================================
  Files          10       10              
  Lines         846      828      -18     
==========================================
+ Hits          747      749       +2     
+ Misses         99       79      -20     

Impacted Files	Coverage Δ
src/table/dataframes_interface.jl	64.81% <ø> (ø)
src/table/dtable.jl	84.44% <100.00%> (ø)
src/table/dtable_column.jl	100.00% <100.00%> (+35.71%)	⬆️
src/table/operations.jl	98.30% <100.00%> (ø)

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[ExpandingMan]

Do you still want to merge this? Thought I'd just bump in case I want to start looking into the package, thanks!

[krynju]

Yeah I am. I think it's even ready to merge.
I'll check tomorrow/Sunday and merge if nothing is left to add

This is just some under the hood changes and code cleanup of the dataframes select functionality (emulating full column functions like mean etc)
This doesn't introduce anything new, but I had to clean this up since I left it in a pretty bad shape 🥲