Collect code coverage

[Luthaf]

This will require a new CI job.

For C and C++ code coverage, we will need to build the code with the -coverage gcc/clang flag; then running the code (from Python or directly) and then collecting coverage with gcov/lcov.

For Python code coverage, we can use the coverage package.

I don't know how to do this for CUDA, @nickjbrowning is there a similar flag for nvcc?

[nickjbrowning]

Code coverage on GPU code (device-side) is not possible as far as I'm aware. Only for the CPU (host) code is this possible.

See here