In general we may want to enforce that a config file is explicitly loaded before any access is requested. However, there are times when this is non ideal behavior. We introduce a compile time flag (CONFIG_HARSH, and CONFIG_WARN). If config hars is defined then a runtime error will be thrown if a config value is requested before the config file has been loaded. If Config warn is defined (and config harsh is not) then a warning will be printed, otherwise nothing will happen. If either warn or nothing is defined this means that the default values defined in the get methods will be used.
Note that the meson build system has had an option added -Dconfig_error_handling=["none", "warn", "harsh"] (default="none") which can be used to manage these compile time options. In general release builds should have this disabled while debug builts should have it set to harsh.
In order to prevent traversing the YAML tree I have added a hash map (O(1) lookup) to cache already accessed config variables. I have also added a vector to store keys requested but not found so we do not need to check for those every time
At many points in the code we may want configurable options, the Config class usses a yaml file to make this easy. It also allows for namespace references "opac:lowtemp:file" etc...
