[FIX] simplified EmpiricalFormula

Removed some methods takings String arguments that blew up the interface.
> I guess this discussion exceeds the current pull request

I agree, @timosachsenberg, I everyone else agrees, can you replace this with 

```cpp
return neutral_loss_diff_formula_.isEmpty() && !neutral_loss_diff_formula_.isCharged();
``` 

and the rest we move to a separate issue.