[WIP] cmake gcovr coverage

[zeratax]

creates a new target to create c++ coverage with gcovr (pip install --user gcovr)

this would be executed with

$ make -C build yacx_coverage

[zeratax]

The issue here is that this requires the tests to be executable with cmake, e.g.:

$ make -C build test

Instead of:

$ pushd build/test/bin
$ tests
$ popd

but this is stuck behind #72

Furthermore CodeCoverage.cmake requires cmake v3.12 (for the find_package(python), but the Uni-Server hasn't been updated in a while and so it's stuck on 3.09. Either we get it updated or use a locally compiled version.