Clean-up dependencies

[20DM]

This change set is to

• make sure the package can be built with Conan v2
• make sure the package can be built with vanilla CMake (+ dependencies from homebrew on MacOS)
• get rid off GreatCMakeCookOff
• upgrades a few dependencies to more modern versions, incl. eigen/3.4.0, catch2/3.4.0, spdlog/1.12.0
• add code tweaks to support interface changes in both catch2 and spdlog
• add code tweaks to get rid off C++ compile time warnings
• harmonise the package name spelling such that it appears as sopt, regardless of whether building with Conan or CMake
• keep the README in sync.

Closes #359 by the looks if it.

[20DM]

This probably needs the cg_cmake_tweaks branch from cppflow to go in first...