Perturb mineral

I wrote a little perturb mineral thing and modified step_3 from the tutorial to example_uncertainties.py to use it. What do you guys think? And what do you think about removing 'err_' from all the uncertainty dictionaries?
I assume the cases with a zero error bar are also caught in the 'try/except'. Or would you like it more explicit, Ian? I know notice the except statement still mentions the 'err_' flag.