String formatting

Summary

The current code base currently uses a mix of stringstream and xDAQ toolbox::toString (and maybe more) in order to format string. The format is unpractical and the latter not type-safe. One should do better.

I'm proposing to use the {fmt} library everywhere. It provides type-safe formatting functions with a syntax close to Python's format. Moreover, its API is compatible with the C++20 std::format API that should be available sooner or later, preventing future maintenance concerns.

What is the expected correct behavior?

Use the same, type-safe, string formatting library everywhere.

Embed {fmt} into the repository and convert all stringstream and toolbox::toString calls to {fmt} calls.