Investigate code test coverage tools

[Walther]

Currently, we have a lot of code that is not tested. Unfortunately, we also do not have easy visibility into which parts are untested.

Let's investigate various tools, e.g.

• codecov
• coveralls
• others?

This would be a tool for figuring out where we are lacking coverage and help us improve things.
This would not be about trying to achieve some specific percentage of coverage as an absolute value.