Implement 100% test coverage.

[RodneyCodess]

Description

This PR updates codes and adds tests to sustain 100% code coverage by using a library Coverage.py.

This PR makes changes, adds extra steps, and overall improves readability to the docs/developing.md to ensure correct procedure.

Motivation and Context

The reason for this PR is to have completely 100% code coverage so that we can confidently change code knowing it won't break anything.

For any code that couldn't be tested, we used # pragma: no cover to bypass the coverage report.

Tests performed

We ran our initial test and did a coverage report to figure out how much code we needed to cover. To completely cover our code, we added more tests and kept doing coverage reports until we finally got to 100% code coverage.

Types of changes

• Refactoring / documentation update (non-breaking change which does not change functionality)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• CHANGELOG.md has been updated
• The milestone is set correctly on the pull request
• The appropriate labels have been added to the pull request
• Running the automated tests (see docs/developing.md) produces no errors
• Updates have been made to the xdmod-notebooks repository as necessary, and the notebooks all run successfully