replace DictTable with DictColumnTable

[OkonSamuel]

DictTable are just the julia Dicts which doesn't preserve ordering. This PR replaces DictTable with DictColumnTable which preseve key ordering.

[OkonSamuel]

@ablaom a review would be appreciated.

[codecov-commenter]

Codecov Report

Merging #59 (4526f8e) into dev (eb29518) will decrease coverage by 0.22%. The diff coverage is 44.44%.

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@@            Coverage Diff             @@
##              dev      #59      +/-   ##
==========================================
- Coverage   92.94%   92.73%   -0.22%     
==========================================
  Files           4        4              
  Lines         397      399       +2     
==========================================
+ Hits          369      370       +1     
- Misses         28       29       +1     

Impacted Files	Coverage Δ
src/NearestNeighborModels.jl	100.00% <ø> (ø)
src/models.jl	91.17% <44.44%> (-0.41%)	:arrow_down: