Add Categorical Likelihood

[sharanry]

Adds categorical likelihood. This can be used along with MOInputs and MOKernels to model multi-class classification.

[codecov-commenter]

Codecov Report

Merging #16 into master will increase coverage by 10.22%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master      #16       +/-   ##
===========================================
+ Coverage   62.50%   72.72%   +10.22%     
===========================================
  Files           3        4        +1     
  Lines           8       11        +3     
===========================================
+ Hits            5        8        +3     
  Misses          3        3               

Impacted Files	Coverage Δ
src/GPLikelihoods.jl	100.00% <ø> (ø)
src/likelihoods/categorical.jl	100.00% <100.00%> (ø)

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[devmotion]

I'm not sure if @willtebbutt's intention was that #18 should be addressed before merging any of the open likelihood PRs?

[sharanry]

I'm not sure if @willtebbutt's intention was that #18 should be addressed before merging any of the open likelihood PRs?

I thought he meant we address common tests before introducing anymore new likelihood PRs. I was waiting for these 2 PRs to get merged before starting on the TestUtils. Maybe I was mistaken.

[devmotion]

I thought he meant we address common tests before introducing anymore new likelihood PRs.

I assume that's what he had in mind, but since I'm not completely sure I think it's good if we wait and let him comment on it.

[willtebbutt]

I'm not particularly fussed what order this happens in, provided that no new PRs are opened without having the test utils sorted :)