Logging / Tracing / Debugging facilities

In order to better understand the internal flow when debugging or developing, it would be very helpful to have run-time switchable logging, including debugging and tracing levels.

For a simple and fast solution, https://github.com/gabime/spdlog could be a good option.
More ready-made C++ logging facilities are [available](https://github.com/fffaraz/awesome-cpp#logging).