Setup a global default spdlog logger

The default spdlog logger uses spdlog's built in registry to store the default logger. The registry's singleton implementation uses a local static variable that causes the default logger to be non unique. https://github.com/gabime/spdlog/blob/v1.x/include/spdlog/details/registry-inl.h#L287

This has negative impliations for Spawn which uses the default logger. We would like to be able to set the log level one time at the initialization of the spawn program such as here https://github.com/NREL/Spawn/blob/compiler/cli/main.cpp#L46.

The problem is that the different compiled units of Spawn have their on log registry, so the log settings established in main do not apply to other contexts. The result is we have to set log settins for each log instance such as here https://github.com/NREL/Spawn/blob/compiler/optimica/optimica.cpp#L20.

The purpose of this issue is to design some type of common log instance that applies across compiled units.