Add test coverage from Coveralls in CI

[rhugonnet]

During CI in GitHub Actions, we now compute the test coverage, upload it to Coveralls, and display the coverage badge on the README. Our current code is covered by tests at 80%, see details at https://coveralls.io/github/GlacioHack/xdem.

In steps:

• We now tell pytest to output the coverage with the --cov option,
• The coverage file is then converted to LCOV format by coveragepy-lcov (needs python>3.8). The issue of creating a LCOV output will be solved soon in pytest: Add support for LCOV output pytest-dev/pytest-cov#536, at which point with can simply pass --cov-report=lcov to pytest,
• We start Coveralls in parallel, using our token, to process and upload the coverage within the job test,
• We finish the job in a separate job finish.

[adehecq]

Great addition! Thanks for getting into this!