Added unique return codes

[fra87]

Added return codes as anticipated in #884

The return codes are now an Enum inheriting from str, so an explicative message can be automatically printed without helper functions.

The return codes are in file returncodes.py

84 new dedicated tests are added to test/core/test_returncodes.py

Documentation has been updated

At the end I run make test, make test_flake8 and make docs. All tests succeeded and flake8 did not report anything. Development and tests were done with Python 3.9.2

[jmcnamara]

I'll admit that I wasn't very enthusiastic about this change, and I am worried that it will break existing check on return values, but you clearly but thought and effort into this so you changed my mind.

I do have quite a few comments to make and if you bear with me through those changes then I'll merge the PR.

[jmcnamara]

You will need to squash the two commits into one.

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[fra87]

I'm happy to hear that; I originally thought it would have been much easier, but there were lots of dependencies (and I wanted to write a complete set of tests).

In any case let me know if there is anything you think is worth modifying.

I squashed the two commits in one, but then there was a conflict. I fixed it properly (actually it was already fixed in the new commit, because the issue was opened before I started working on the modification), but then the merge commit appeared and I'm not sure I can squash also the merge commit. How can this be solved? Can the two commits be pulled, or do you think it is better to start from scratch on a new branch generated from the new master, copy everything from the old branch in the new and then commit?

[jmcnamara]

is better to start from scratch on a new branch generated from the new master, copy everything from the old branch in the new and then commit?

I've done that in the past when a project I was trying to submit to had moved on and created a conflict. Feel free to close this PR and submit a new one without conflicts.

[fra87]

Closing PR because PR #892 was created (including all the modifications to master up to today)