Added SimpleCalculator

[Harish5232]

I havecreated a file Calculator and added SimpleCalculator.java file at src/test/java/com/thealgorithms/maths/Calculator/SimpleCalculator.java.

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• All filenames are in PascalCase.
• All functions and variable names follow Java naming conventions.
• All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• All new code is formatted with clang-format -i --style=file path/to/your/file.java

[DenizAltunkapan]

I don't think a calculator really fits the scope of an algorithm repository. Whats your opinion abt this? @alxkm

[Harish5232]

I know its not a typical algorithm or follows any data structure but can be a beginner friendly program/code. So i have created a maths folder and inserted the file there so it can be kept seperate from other files.

[alxkm]

@DenizAltunkapan, I agree. Also, there already exists some kind of calculator:
https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/maths/MathBuilder.java