Add merge strategies for maps

[bvenn]

Thank you for contributing to FSharp.Stats. Please take the time to tell us a bit more about your PR.

Please reference the issue(s) this PR is related to

#263

Please list the changes introduced in this PR

• Addition of Map.merge, Map.mergeBy, Map.mergeSubtract, and Map.mergeAdd
• Addition of Frequency.merge, Frequency.subtract, and Frequency.add
• Addition of Empirical.merge, Empirical.mergeBy, and Empirical.add
• Addition of Distribution.Empirical documentation

Description

Maps are the foundation of modules Frequency and Distribution.Empirical. Frequency maps can be merged of data histograms should be combined. Generic mergeBy functions enable to decide what happens when keys are present in both input maps.

[Required] please make sure you checked that

• The project builds without problems on your machine

[Optional]

• Added unit tests regarding the added features

[codecov-commenter]

Codecov Report

Merging #264 (814ae18) into developer (3d6a220) will increase coverage by 0.08%.
The diff coverage is 61.22%.

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@@              Coverage Diff              @@
##           developer     #264      +/-   ##
=============================================
+ Coverage      46.41%   46.49%   +0.08%     
=============================================
  Files            152      153       +1     
  Lines          15255    15304      +49     
  Branches        1980     1983       +3     
=============================================
+ Hits            7080     7115      +35     
- Misses          7628     7641      +13     
- Partials         547      548       +1     

Impacted Files	Coverage Δ
src/FSharp.Stats/Distributions/Frequency.fs	0.00% <0.00%> (ø)
src/FSharp.Stats/Distributions/Empirical.fs	24.41% <33.33%> (+0.96%)	⬆️
src/FSharp.Stats/Map.fs	66.66% <66.66%> (ø)
tests/FSharp.Stats.Tests/DistributionsEmpirical.fs	100.00% <100.00%> (ø)