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?
Here I make a backup of the mineral parameters... not an extremely elegant solution. Does anybody have better ideas? I slightly prefer the model of defining a composite and then being able to perturb it n times. But if we want to go with the model where perturb is in init and you have to initialize n composite, I could also go with that.