Loading: Improve code flexibility to load "daily" frequency datasets

The current seasonal code builds the path to the files that need to be loaded using the frequency specified in the recipe: monthly_mean or daily_mean.

But, for some variables or for datasets outside of /esarchive, the path may be built in a different way, or it may have a different name such as daily. It would be good to try to change the structure of the archive and the code in load_seasonal() to improve this and make it more flexible.