XPath predicate parsing requires string value

[remingtonc]

Per #40 XPath predicate exprs currently must be strings. Must investigate:

1. Current XML XPath regex result, does it require a string?
2. May YANG XPath predicate values be types other than strings?
3. Easy fix, then scope something more robust like a state machine. pyang just added some xpath code, might be able to disaggregate.