Add code coverage to stdlib.

[loiseaujc]

Following the discussion in #937, this PR (tries to) add code coverage to stdlib using codecov.

Key facts

• Added a new code-coverage.yml Github Action which runs on every push and pull requests.
• Uses fpm to run the test with the added -coverage flag added for compilation with gfortran.
  • Use the release profile with xdp and qp turned on.
• Automatically upload the coverage report to codecov
• Requires a CODECOV_TOKEN to be set up in the settings of the repo. This should be done by the one of the org owners I believe.

Current issue

The Github Action runs correctly and the coverage report is upload to codecov (see here). Note however that it does not seem to include the coverage report for a number of modules including the linalg for instance. I'm not sure what's going here and I have a fairly limited experience with gcov and lcov.

I get warnings like these

geninfo: WARNING: WARNING: GCOV did not produce any data for /home/runner/work/stdlib/stdlib/build/coverage/src_stdlib_sparse.f90.gcno	(use "geninfo --ignore-errors gcov,gcov ..." to suppress this warning) 

despite the test for sparse being run.

ping @perazz, @jalvesz and @jvdp1

[codecov]

Welcome to Codecov 🎉

Once you merge this PR into your default branch, you're all set! Codecov will compare coverage reports and display results in all future pull requests.

Thanks for integrating Codecov - We've got you covered ☂️

[loiseaujc]

I've ended up chaining the code coverage thing with the fpm-deployment action and it now works like a charm. The code coverage is generated from the stdlib-fpm branch with xdp and qp turned on (see here). It seems like we're slightly below the 25% mark.