Improving CalculatorDevops

[michailidm]

Hello, I would like to contribute to your project.
The changes I made are the following:

• more exception handling so that the program would not crash in case of wrong input
• added more operations (sin, cos, tan, square root, exp)
• split the program in classes so it would be more extensible
• added the option of exiting the program
• removed .0 ending from integers (e.g. 4.0 is now printed as 4)
• added parentheses where needed (e.g. 3 - (-2))
• added more tests
• some other small changes

If you approve my work, please merge my PR.