Improve readability and test coverage of cli

[gmmajal]

Relevant issue: #40

Background info: # any relevant background info for additional context, references to documentations etc.

• Our codecov plugin is highlighting that the most amount of lines missed by our unit tests reside in the qualle/interface/cli.py script. We can improve our code coverage by adding more unit tests for the cli interface.
• The cli_entrypoint() method is too long. It should be split into smaller chunks in order to make it easier to test and to improve readability.

Changes introduced: # list changes to the code repo made in this pull request

• Additional unit tests have been inserted in tests/interface/test_cli.py. These tests check the behavior of the method cli_entrypoint() and other methods in qualle/interface/cli.py which were previously missed in our coverage.
• cli_entrypoint() has been refactored to make it easier to read. Four different methods have been added to create subparsers which are being utilized by our cli.
• main.py has been refactored slightly in order to write appropriate unit tests. The coverage is skipped for the line under the if block. Some new unit tests have been under the script test_main.py

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (420d37e) to head (d7f917e).

Additional details and impacted files

@@             Coverage Diff             @@
##           master       #42      +/-   ##
===========================================
+ Coverage   94.87%   100.00%   +5.12%     
===========================================
  Files          23        22       -1     
  Lines         898       905       +7     
===========================================
+ Hits          852       905      +53     
+ Misses         46         0      -46     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[gmmajal]

Further changes:

• test_main.py has been removed. Furthermore, test coverage for main.py is omitted in the pyproject.toml file. This choice is used as opposed to #pragma: no cover. It was recommended here: https://stackoverflow.com/questions/5850268/how-to-test-or-mock-if-name-main-contents, since it avoids clutter inside the code base.