adding coverage reports

How to use: cmake -DCMAKE_BUILD_TYPE=Coverage <source-dir>, cmake --build . --target install in a configured build directory, then from the build folder run <source-dir>/Tools/coverage.sh, which is also available as <install-dir>/share/tools/coverage.sh

If all goes well, this generates the folder coverage-report in your local directory, with a html report on coverage. Works on my computer.