Add gcov report

[zonmen]

• Add gcov report to ./tests/code_coverage/report and use file index.html.
• Update README.md
• Update CMakeLists.txt To use this feature, you need to install gcovr: $sudo apt-get install gcovr And make target code_coverage $cd build $make code_coverage Or if you use vs code cmake extesion, choose special target here and then build:

[zonmen]

• Disable in cmake generation gcov report on machine, that doesn't have gcovr app. Like github virtual machine.

[ValentinSidorov]

Great job.