Add Euler Modular Inverse

[GnsP]

Description of new feature, or changes

Implements function to calculate modular inverse using euler's theorem,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