Uploading coverage results as part of CI

[RobPasMue]

Resolves #270

[RobPasMue]

Coverage report is now generated in HTML format and uploaded as an artifact to the GitHub Actions workflow, as it can be seen in the following screenshot.

Once the project goes public and it makes use of codecov.io, we will be capable of including the corresponding badge in the README.rst file (other solutions imply committing a new README file in each workflow action where the coverage badge would be updated. Though it is possible, it is not a recommended solution - auto updates of files can lead to an enormous amount of useless commits).

If you download the file, now you can access the line-by-line coverage and a summary of the results. I have also excluded those files configured in the .coveragerc of the project (which was apparently not being used). Find below some examples of the outcoming HTML coverage report:

Please feel free to add any suggestions :)