Add unit tests for interval

[bvenn]

Fixes #208

Please list the changes introduced in this PR

• add interval documentation
• add interval test
• fix interval nan handling

[Required] please make sure you checked that

• [x] The project builds without problems on your machine

[Optional]

• [x] Added unit tests regarding the added features

[codecov-commenter]

Codecov Report

Merging #227 (54eb168) into developer (d0fbe24) will increase coverage by 0.88%. The diff coverage is 87.50%.

@@              Coverage Diff              @@
##           developer     #227      +/-   ##
=============================================
+ Coverage      40.46%   41.34%   +0.88%     
=============================================
  Files            125      126       +1     
  Lines          12990    13197     +207     
  Branches        1787     1792       +5     
=============================================
+ Hits            5256     5456     +200     
  Misses          7287     7287              
- Partials         447      454       +7     

Impacted Files	Coverage Δ
src/FSharp.Stats/Intervals.fs	13.95% <0.00%> (+12.60%)	:arrow_up:
src/FSharp.Stats/Rank.fs	42.64% <0.00%> (ø)
tests/FSharp.Stats.Tests/Main.fs	0.00% <0.00%> (ø)
tests/FSharp.Stats.Tests/Interval.fs	97.42% <97.42%> (ø)

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