Check for Heterogram in String

# Introduction

In this lab, we are tasked with writing a program that checks if a given string is a heterogram or not. A heterogram is a word, phrase, or sentence in which no letter of the alphabet occurs more than once. We will define a function called `is_heterogram()` that takes a string as input. The function will return `Yes` if the string is a heterogram and `No` otherwise.