Reduce the computational expenses of the automated GitHub Workflows

[mpvanderschelling]

The problem

The automated GitHub workflows need some modification to address the following

• We should minimize the number of minutes that are used with GitHub Workflows so that we don't reach the limit before the end of the month.

As the bessagroup account is shared, we should be mindful about this
At the moment, if a pull request is active on either one of the pr/** branches or the main branch, everytime a commit is made, the automated tests are run. These is still a skimmed down version of the tests (only run on Linux + 1 Python version), but it is redundant to run it for every commit.

Proposal

Automated tests should be run only in the following scenarios:

• creation of pull request from issue branch to pr/** (2)
• creation of pull request from pr/** to main (1)
• about to merge to from issue branch to pr/** (2)
• about to merge to from pr/** to main (1)
• after merging to main (all tests to show reliability of the code to users) (1) (3)

How the 'about to merge' trigger might be implemented is still under investigation.

Type of tests

Without going in to much detail, we consider the following test suites:
(1) full test suite (all tests, all major OS, Python 3.7+)
(2) smoke tests (only computationally inexpensive tests, only Linux, Python 3.8)
(3) building sphinx-doc and committing to gh-pages branches for GitHub pages

[mpvanderschelling]

@BernardoFerreira ; can you take a look at the problem description and add anything that you think might be necessary to address this issue ?