Consider using a code coverage service

[bouweandela]

At the moment, it is not clear to me as a contributor which parts of the code that I am contributing are covered by tests, so I don't know if can safely change things or if I'm silently breaking something. I guess the same goes for pull request reviewers.

It would be convenient to

1. Explain in the contribution guidelines how to generate code coverage metrics
2. Use a code coverage service that reports code coverage (changes) on every pull request. Popular examples of such services are Codecov and Coveralls

@SciTools/iris-devs Is this something you have considered before? Does it sound interesting?

[trexfeathers]

#4765

@lbdreyer

[bouweandela]

That looks great! Are there any plans to finalize that pull request?