Improved control over code changes

[jesper-friis]

Description

Implemented the formalised code changes described in PR #920.

Benefits: Make it possible to improve our control over code changes. Provide predictability for when changes will happen and give users a time-span to a given versions number to adapt to new changes.

Motivation: This PR was motivated by PR #907. When merged, we can address PR #907 as our first behavior change.

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Possible further work: Automatic generation of a page in the online documentation that lists upcoming versions including (for each upcoming version):

• deprecated features that are planned to be removed
• new behavior will be enabled by default
• old behavior that is planned to be removed

Would such a page be useful? It would not include planned new features (for that we can use github issues and milestones).

Type of change

• Bug fix & code cleanup
• New feature
• Documentation update
• Test update

Checklist for the reviewer

This checklist should be used as a help for the reviewer.

• Is the change limited to one issue?
• Does this PR close the issue?
• Is the code easy to read and understand?
• Do all new feature have an accompanying new test?
• Has the documentation been updated as necessary?