Xml doc comments and tests for discrete padRnd.

[OwenMadden]

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Part of #281
Filled in xml doc comments for Discrete.padRnd function. Added code example and test for example.

A few sentences about your contribution
Filled in xml doc comments for Discrete.padRnd function. Added code example and test for example. Test that the padded data contains data taken from the original data set.

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

[Optional]

• [Yes] Added unit tests regarding the added features

[codecov-commenter]

Codecov Report

Merging #307 (28106c7) into developer (e4fd26a) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@              Coverage Diff              @@
##           developer     #307      +/-   ##
=============================================
+ Coverage      46.73%   46.74%   +0.01%     
=============================================
  Files            148      148              
  Lines          16225    16230       +5     
  Branches        2196     2196              
=============================================
+ Hits            7582     7587       +5     
  Misses          7987     7987              
  Partials         656      656              

Files	Coverage Δ
src/FSharp.Stats/Signal/Padding.fs	0.00% <ø> (ø)
tests/FSharp.Stats.Tests/Signal.fs	98.74% <100.00%> (+0.04%)	⬆️