Use std::function for the global error handler

Fix signed/unsigned mismatch in VS

Fix C++20 build resulting in deprecated implicit copy assignment operator warning

Allow compilation with nvc++ (and possibly PGI)

add macros for overriding the individual level names

Revert "added spdlog::level::set_string_view to enable alternate log level names without changing the build via SPDLOG_LEVEL_NAMES"

This reverts commit 2a16d1d2.