[FIX] simplified EmpiricalFormula

Removed some methods takings String arguments that blew up the interface.
In principle: yes - except one wants to model e.g. a single electron charge.
I think it would be a good idea to enforce this invariant.
The problem that I see in general with this class is that it is highly targeted to proteomics / proton charges. It doesn't fit well with our metabolomics integration and I think I remember some places where some strange calculations are done to e.g. subtract an electron mass. Maybe @aiche and @cbielow have a better understanding of it. Would it make sense to generalize this class to support arbitrary charges (e.g. from adducts)?