Add special functions tests

[kMutagene]

This PR will add tests for the following special functions:

• [x] Binomial
• [x] Erf
• [x] Factorial
• [x] Logistics
• [x] Gamma
• [x] Beta

implements #215 for erfcx, gamma, gammaLn, beta, betaLn, factorial, factorialLn,Binomial.coefficient, Binomial.coefficientLn

[codecov-commenter]

Codecov Report

Merging #207 (8582dd7) into developer (262f1ac) will increase coverage by 0.57%. The diff coverage is 96.22%.

@@              Coverage Diff              @@
##           developer     #207      +/-   ##
=============================================
+ Coverage      31.61%   32.19%   +0.57%     
=============================================
  Files            124      124              
  Lines          11664    11707      +43     
  Branches        1784     1786       +2     
=============================================
+ Hits            3688     3769      +81     
+ Misses          7535     7494      -41     
- Partials         441      444       +3     

Impacted Files	Coverage Δ
src/FSharp.Stats/SpecialFunctions/Logistic.fs	100.00% <ø> (+100.00%)	:arrow_up:
tests/FSharp.Stats.Tests/Main.fs	0.00% <0.00%> (ø)
src/FSharp.Stats/SpecialFunctions/Erf.fs	96.42% <75.00%> (+57.96%)	:arrow_up:
src/FSharp.Stats/SpecialFunctions/Factorial.fs	88.88% <100.00%> (+88.88%)	:arrow_up:
tests/FSharp.Stats.Tests/SpecialFunctions.fs	100.00% <100.00%> (ø)
src/FSharp.Stats/Fitting/Spline.fs	91.17% <0.00%> (-0.26%)	:arrow_down:
src/FSharp.Stats/Array.fs	0.00% <0.00%> (ø)
...tats/ML/Unsupervised/PrincipalComponentAnalysis.fs	0.00% <0.00%> (ø)
src/FSharp.Stats/Ops.fs	4.00% <0.00%> (+0.15%)	:arrow_up:
... and 2 more

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[kMutagene]

@bvenn as this now also changes code usage of special functions in the distribution modules, i'd suggest to merge this first and rebase all other PRs from the hackathon that fix code on this. Double check for correct usage of e.g. _gamma instead of gamma in new commits.