add euler totient function, issue #10

[GnsP]

Description of new feature, or changes

Adds Euler totient function (phi) as required in issue #10 .

Checklist

• [x] Your branch is up-to-date with the base branch
• [x] You've included at least one test if this is a new feature
• [x] All tests are passing

Related Issues and Discussions

issue #10

People to notify

@manrajgrover