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...
