Don't store models and images to home

[cswaney]

We want to make a clean separation between the location where Blackfish application data is stored, BLACKFISH_HOME_DIR, and the location where models and images are stored, BLACKFISH_CACHDE_DIR.

It may even make sense to separate the location where models are stored from the location where images are stored, because users might be using a shared image location and a personal model directory.