Restricted Rockfalls to the Volcano

Removed some Instantiate calls to restrict Rockfalls to the volcanic
apocalypses.
Also fixed a line where the Volcano apocalypses were setting themselves
to be ApocalypseType.Desert.
Made the weather effects Modular

Moved the sandstorm to a script, added an Ash effect.
Merge branch 'master' of https://github.com/kvelury/apocalyptia