Use codecov to report test coverage

[timcallow]

We could integrate codecov into the testing workflow. This would allow us to separate the passing of the CPU tests from ensuring coverage thresholds are met, besides giving more detailed coverage information. We could also add a nice badge to show our coverage %, but this maybe should wait until we add more tests to increase the coverage to at least ~80%.

[RandomDefaultUser]

Am I understanding https://about.codecov.io/pricing/ correctly that for an open source public repo we can essentially get everything with the free plan? Or how did you cope with it technically not being free/open source when setting it up with atomec?

[timcallow]

Yeah, I never noticed anything missing with the free plan for atoMEC - I don't forsee any issues there