Use CategoricalDistributions for UnivariateFinite, instead of MLJBase

[ablaom]

This PR needs:

• MLJBase 0.19 released
• Version updated to 0.2.0

In this PR, we:

• Add CategoricalDistributions.jl as a dependency
• Bump compat for MLJBase to 0.19 and address the fact that UnivariateFinite functionality is not defined in MLJBase 0.19, but in CategoricalDistributions.jl instead.
• (breaking) Change syntax EnsembleModel(atom=...) to EnsembleModel(model=...) for consistency with other MLJ model wrappers (eg, TunedModel) but additionally allow passing model as non-keyword argument, as in EnsembleModel(my_tree, ...).

[codecov-commenter]

Codecov Report

Merging #13 (da40e11) into dev (7ca66d7) will increase coverage by 8.04%.
The diff coverage is 70.00%.

@@            Coverage Diff             @@
##              dev      #13      +/-   ##
==========================================
+ Coverage   70.46%   78.50%   +8.04%     
==========================================
  Files           2        2              
  Lines         237      214      -23     
==========================================
+ Hits          167      168       +1     
+ Misses         70       46      -24     

Impacted Files	Coverage Δ
src/MLJEnsembles.jl	100.00% <ø> (ø)
src/ensembles.jl	78.40% <70.00%> (+8.06%)	⬆️

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