Fixes #320 ,The zero division error and added a new power button to calculate power

[Atharv-Joshi]

Description

A short summary of what is included in your Pull Request.

The calculator.py file has been updated and the zero division error has been solved.
Now a popup is shown to the user instructing him/her to correct the input instead of displaying the error message on the terminal.

A new 'power' button is added which calculates a ^ b .

The output.png is respectively updated and contains the screenshot of the new UI.

The readme.md is also updated with the new screenshot

Working Screenshot :

Fixes #320

Type of change

Choosing one or more options from the following as per the nature of your Pull request.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Documentation Update

Checklist:

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• I have named my files and folder, according to this project's guidelines.
• My code follows the style guidelines of this project.
• My Pull Request has a descriptive title. (not a vague title like Update index.md)
• I have commented on my code, particularly in hard-to-understand areas.
• I have created a helpful and easy to understand README.md, according to the given README_TEMPLATE.
• I have included a requirements.txt file (if external libraries are required.)
• My changes do not produce any warnings.
• I have added a working sample/screenshot of the script.