Make it possible to use external logger and wrap logger calls

[vvolkl]

The papas repository includes the source code for spdlog. It makes much more sense to try to find an spdlog installation via find_package, and fall back to the local code (or use std::iostream) if not found (since the point of using the library is mostly to use papas logging in an external framework). For this the logging should be wrapped in some implementation independent macros.