Updating the README.md to point to the file USING_COLLABORATING.md (renamed and adapted from COLLABORATION_AGREEMENT.md). Add a file in the FLUKA directory pointing to their website for license, mentioning the desired reference and the request to register in case people are using a version with binary enclosed. Changes from GPL to BSD in README.md and USING_COLLABORATING.md to be made in another merge request.

Port C8 logging infrastructure to refactor.

Move implementations to .inl file in detail.

Delete commented out inline definition.

Switch to snake case and move default logger.

Remove explicit set_level methods.

Move default pattern into corsika::logging.

Remove superfluous comments and fix references.

Remove macros from .inl file.

Rename macros from C8LOG to CORSIKA_LOG

Remove double import of spdlog into corsika::logging