Adds the student-t likelihood and a few more changes

[theogf]

• Adds the Student-T likelihood as a scale-mixture of Gamma
• Define a new type compatible with the ntmean and ntrand functions. This helps unifying existing distributions and created ones under one hood

[codecov]

Codecov Report

Merging #10 (ab6dc5f) into main (66eeaa0) will increase coverage by 0.48%.
The diff coverage is 92.30%.

❗ Current head ab6dc5f differs from pull request most recent head 4640e56. Consider uploading reports for the commit 4640e56 to get more accurate results

@@            Coverage Diff             @@
##             main      #10      +/-   ##
==========================================
+ Coverage   93.09%   93.57%   +0.48%     
==========================================
  Files           8       10       +2     
  Lines         304      358      +54     
==========================================
+ Hits          283      335      +52     
- Misses         21       23       +2     

Impacted Files	Coverage Δ
src/SpecialDistributions/SpecialDistributions.jl	60.00% <50.00%> (-40.00%)	⬇️
src/SpecialDistributions/polyagamma.jl	91.96% <50.00%> (+1.28%)	⬆️
src/SpecialDistributions/ntdist.jl	90.47% <90.47%> (ø)
src/likelihoods/studentt.jl	94.59% <94.59%> (ø)
src/SpecialDistributions/polyagammapoisson.jl	100.00% <100.00%> (+8.33%)	⬆️
src/TestUtils.jl	96.36% <100.00%> (+0.06%)	⬆️
src/generic.jl	80.00% <100.00%> (+3.07%)	⬆️
src/likelihoods/bernoulli.jl	100.00% <100.00%> (ø)
src/likelihoods/poisson.jl	100.00% <100.00%> (ø)
src/utils.jl	100.00% <100.00%> (ø)
... and 1 more

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