Proposal: Using logging library spdlog

Currently we are using different kind of logging functionalities: Konstantin mostly uses the old basics/UbLogger. Martin mostly uses the basics/Core/Logger I wrote some years ago. In-between there are a lot of cout/printf.

My proposal; instead of agreeing upon one of those, we completely switch to the established, free, open-sourced, function-rich and heavily used in many other C++ libraries spdlog.

This library is header-only and could easily integrated in our software. For further information and usage examples please take a look at https://github.com/gabime/spdlog

What do you think?