Consider replacing logger with spdlog

[jdumas]

spdlog is an awesome logging library offering tons of feature. It is super lightweight as well, thread-safe, etc. We use it in Lagrange, where it is exposed as a single logger object, which under the hood uses a global static logger.

Using spdlog would simplify integration of geogram in third-party codebase. It has a very well-designed API and offers lots of ways to control logger sinks and redirect logging messages where we need them.