First implementation of conan for dependencies.

This is a first pass of using Conan for our dependency management; this is the simplest possible Conan setup and we can continue to configure this using the fancier features of Conan as needed.

The current Conan setup provides:

• Eigen
• spdlog
• catch2
• boost (even though it isn't explicitly used yet)
• zlib (which is a dependency for cnpy right now)

conan is usually a separate external step (i.e. conan install . to install dependencies). I've hooked conan into the CMake build process so the build instructions stay the same as they are currently.

To add a dependency to a particular library provided by conan, you link against the target as so target_link_libraries(CORSIKA CONAN_PKG::eigen).