Structured logger for solvers

• the TNL::Logger class should be structured, i.e. like the inverse of the ParameterContainer
• then it will be possible to make serialization classes (JSON, yaml or pretty-format) or let it pass to Python
• idea: the writeProlog method just serializes ParameterContainer into Logger, it should be possible to do that automatically
• some logging library like spdlog or Easylogging++ could be used

(moved from #26 (closed))