VectorComposite::getStructure() sorted string

To easily check, compare, etc. VectorComposite should have the method getStructure() however, the ordering in the unordered_map and the string provided by the user does necessarily follow the same order.

So I propose to return a sorted (alphabetically) string. And agreeing on the Wolf::Structure are a sorted, maybe a class instead of a typedef is required here, I don't know.

Opinions? @jsola