[WIP] Logging system

[sykhro]

Preliminary logging system implementation, posting here for comments/suggestions. I went for fmtlib directly instead of spdlog because we needed a wrapper anyway and also because it can be used to replace usage of std::stringstreams, which are far less pretty.

TODO:

• [x] Basic structure
• [x] Utility macros
• [x] Sink naming (useful for internal management, avoids having to do weird things with pointers)
• [x] Per-sink log level filtering
• [x] CLI sink
• [x] CLI: replace usages
• [x] CLI: timestamping
• [x] tests: replace usages
• [ ] Proper CMake checks
• [x] Document the new classes decently
• [ ] File sink (log to file)

side note: fmtlib was not added via git-subtree in order to avoid useless bloat

[AndreaIdini]

Since we are already using gtest, which advantages have fmtlib over glog?

[sykhro]

fmtlib is a generic formatting library which is useful for all sorts of things other than logging (much faster than stringstreams, allows for python-like string manipulation); what replaces glog here is the new Logger class, which should be lighter and functionally equivalent (also easier to integrate with a future gui)

[GabrielePisciotta]

This pull request introduces 1 alert when merging 810375a05f58845597485ac50326f1b82648a441 into cd4b913dd0b29c0e8cd4a25567305113ab0191cb - view on LGTM.com

new alerts:

• 1 for Missing return statement

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