algorithm: signum

[SpiderMath]

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Describe your change:

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new JavaScript files are placed inside an existing directory.
• All filenames should use the UpperCamelCase (PascalCase) style. There should be no spaces in filenames.
  Example:UserProfile.js is allowed but userprofile.js,Userprofile.js,user-Profile.js,userProfile.js are not
• All new algorithms have a URL in its comments that points to Wikipedia or other similar explanation.
• If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

This PR is to add the implementation of the signum function. TheAlgorithms entry is here

[appgurueu]

Looks fine; note: this will currently return 1 for NaN; returning NaN would be more sensible IMO.

[SpiderMath]

Oh yes. I completely missed that case. I'm sorry. Thanks for pointing it out, I'll fix it right away.

[appgurueu]

You still have an else in each branch. Generally using else-ifs even if you are returning on each branch is not necessarily bad practice because it may help improve readability (so I'm fine with it either way), but in this particular example, your ifs are simple enough that you should just get rid of the redundant elses as the returns already do the job.