Support fmt format and other changes

[rbx]

• Introduce two new log calls:
  • LOGF(severity, ...) The arguments are given to fmt::format and the result is logged, e.g. LOGF(info, "Hello {}!", "world");
  • LOGP(severity, ...) The arguments are given to fmt::printf and the result is logged, e.g. LOGP(info, "Hello %s!", "world");
• Use fmt internally instead of ostringstream (where applicable) and std::localtime for better performance.
• New log call LOGN(severity) that outputs an empty line (ping @fuhlig1).
• New log call LOGV(severity, verbosity) that allows to override current verbosity just for the current call.
• Various small optimizations.

The fmt library is bundled with the code (version 5.3.0), but users can override this by providing an external fmt installation via -DUSE_EXTERNAL_FMT=ON -DFMT_ROOT=/fmt/location. If fmt is external, projects that use FairLogger need to find fmt.

@ktf Solves #16.

[ktf]

Cute! Thanks.