Refactor #2 Lazy Classes
- Got rid of objects of SubtractNumbers.java, ConvertLetter.java, MultiplyNumbers.java, DivideNumbers.java in CalculatorInterface.java
- Moved functionality of each of those above classes into CalculatorInterface.java

Refactor #3 Long Method
- Extracted parts of start() within CalculatorInterface.java and formed new methods addition(), subtraction(), multiplication(), and division() within CalculatorInterface.java
- Placed extracted parts into their respective methods.