set equation of state automatically in minerals

Fixes #40 .
Automatically sets the eos for minerals when defined. I took a lot of the set_methods out of the examples.
This also slightly resolves not seeing the method strings to much (Issue #8). However, it has made it more explicit what a mineral database looks like. I think the parser Ian and Bob wrote can easily be adapted. But I also think it would be an important part of the new manual. I can easily see people run into issues when they don't have the right format.