Fix xpath, improve pepxml.roc_curve()

[alephreish]

• Fixes xpath bugs in pepxml.roc_curve() that caused a fatal error [XPathEvalError: Invalid predicate]
• Adds "charge" and "tag" keys to the output to make the function usable in real world.

[levitsky]

Thank you! This is a legacy function and to be honest I don't think I've ever used it myself.
One minor question: should we maybe convert charge to an integer?

[alephreish]

Hi! I wrote my own parser for pepxml in node.js, but recently decided to switch to pyteomics for everyday use - that's when I discovered also roc_curve().
No, "charge" has to remain string as it is a categorical variable and will be used as such in most of the cases. It goes: "all", "2", "3" and "4".

[levitsky]

Ah, OK, thanks.

I'm glad you've found pyteomics to be of use and thank you for the contribution!