Generate coverage report during CI tests

[danielfromearth]

(As part of ncompare's PyOpenSci review (issue #146))

And add a badge — showing the coverage — to the readme

[danielfromearth]

Perhaps use this Action: https://github.com/marketplace/actions/pytest-coverage-comment

[danielfromearth]

Use Codecov, like other NASA projects (https://codecov.io/gh/nasa).