Add distance metrics tests and small fixes to distance metrics

[fejung]

Changes to the distance metrics scirpt:

• NaN excpetion rule in the cityblockNaN for inputs in the form of vectors was missing; this resulted in giving out nan if one of the values of the vector was nan. Added:
  if not (nan.Equals (x)) then
• the function min3 only works with integers, but currently accepts every form of input. Added:
  type annotation int behind the function parameters
  removed inline

Added unit tests for the functions in the distance metrics script:

• tests for the euclidian distance with sequences, vectors, or arrays as input
• tests for the cityblock distance with sequences, vectors, or arrays as input
• tests for the Levenshtein distance with strings as input

Added the respective lines to run the tests to the main.fs script in the tests folder

[bvenn]

• the function min3 only works with integers, but currently accepts every form of input. Added:
  type annotation int behind the function parameters
  removed inline

The function should've worked for any input of type 'IComparable but as this function is private and only utilized as int-comparison helper, it doesn't matter at this position.

[codecov-commenter]

Codecov Report

Merging #210 (261b0b8) into developer (262f1ac) will increase coverage by 1.91%.
The diff coverage is 96.28%.

@@              Coverage Diff              @@
##           developer     #210      +/-   ##
=============================================
+ Coverage      31.61%   33.53%   +1.91%     
=============================================
  Files            124      125       +1     
  Lines          11664    11924     +260     
  Branches        1784     1785       +1     
=============================================
+ Hits            3688     3999     +311     
+ Misses          7535     7480      -55     
- Partials         441      445       +4     

Impacted Files	Coverage Δ
src/FSharp.Stats/DistanceMetrics.fs	46.08% <0.00%> (+46.08%)	⬆️
tests/FSharp.Stats.Tests/Main.fs	0.00% <0.00%> (ø)
tests/FSharp.Stats.Tests/DistanceMetrics.fs	100.00% <100.00%> (ø)
src/FSharp.Stats/Fitting/Spline.fs	91.17% <0.00%> (-0.26%)	⬇️
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%)	⬆️

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