Use integrate() instead of add_relation()

[DanielArndt]

add_relation() is deprecated and will be remove in the future. This removes the warning log.

Also, assert assumptions instead of ignoring the type checker.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that validate the behaviour of the software
• I validated that new and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have bumped the version of the library