Add new enum `loglevel` to `constants.hpp`
this enum is defined for namespace `se2::util`.
Add `logging` class
To access the logging functionality, use the defines from `logging.hpp`.
The available defines are `LOG_TRACE(msg)`, `LOG_DEBUG(msg)`, `LOG_WARNING(msg)`,
`LOG_ERROR(msg)`. You should avoid calling `logging::log()` directly.