Notify user of or report cache size

[pcolange]

Digest uses a cache directory to store models mostly after using the freeze feature. On Windows, this is typically C:\Users\username\AppData\Local\digest\digest\Cache. Depending on how used this feature is and the size of the models, the cache can grow large. It would be nice if the GUI provided the user info on the size of this cache and offer the ability to clean it out to save on disk space. One option is to display this in the info dialog. Another option is to introduce a settings window to enable more control over these types of features.