Iceshield

The "ice-shielding" functionality does not break the code, and has no obvious adverse effects on the model as a whole. Given that it could easily be switched off or removed (specifically, by commenting out or deleting 4 separate lines of code), and that it is not interfering with other parts of the model, this iceshield branch is better off merged with the betterperma branch, which contains all the major science updates to the "base" model of the master branch.

In fact, the betterperma branch would better be named dev from now on, and the master branch should be renamed base. All further new science features should be based on the dev branch. Once the model is "production-ready" (or published), a new master branch should be created then.