maintenance update

[strasdat]

The goal of this PR is to make Sophus future-proof and easier to maintain. In particular, it removes outdated code, updates to more modern workflows, and incorporates newer toolchains and dependencies, hopefully without causing any major backwards compatibility issues.

• Largely simplifies logging and formatting by removing optionality and reverting to an earlier approach for log formatting.

  • The choice between default and BASIC_LOGGING, as well as the libfmt dependency, caused various issues and confusion. FMT warning when not found #520, SOPHUS_USE_BASIC_LOGGING=ON compilation error #516, and SOPHUS_FMT_ARG causes compilization error when SOPHUS_USE_BASIC_LOGGING=ON #436 are just the latest examples.
  • This PR essentially reverts adding fmt dependency #216 from five years ago. While this revert is a significant change, it should not cause any major issues. It merely removes the libfmt dependency and eliminates the need for the BASIC_LOGGING option. Fingers crossed, I did not overlook anything here...

• Newer toolchains and dependencies

  • Moved from C++14 to C++17. This allows the use of std::optional in the API and eliminates the need for a hand-rolled optional type.
  • Eigen 3.4
  • New version of Ceres, HEAD of master (for CI)

• CI

  • Larger overhaul of the main.yml CI action
    • Addition of several newer targets such as ubuntu-24.04 and macOS-14
    • New "install_tests" which test a "sudo make install" followed by an example target "linking" against the installed Sophus library
    • Example usages in CI of the SOPHUS_ENABLE_ENSURE_HANDLER, which has been around for a long time but is largely unknown. This is particularly useful for those who prefer to throw exceptions on ENSURE failures instead of a panic (e.g., Sophus ensure calls abort() #533)
  • (Re)-enable pybind CI - though it is currently failing, see Cannot compile sophus_pybind in Ubuntu 20.04 #537

• Removed old/outdated code:

  • sophus/ceres_local_parameterization.hpp, which is not supported by newer Ceres versions anymore and is superseded by sophus/ceres_manifold.hpp
  • Experimental sophus/velocities.hpp

• Remove hosted Sophus docs folders/toolchain

  • The actual Sophus 1 docs have not been working/hosted for several years now, and re-enabling/maintaining them is too large of a burden...

• Modern format and linting using pre-commit

• Content format and update to the top-level README