Adding containsRefName to SequenceDictionary

(A little update, I'm separating this out and filing it as a separate pull request from a larger request which will follow from us here at GenomeBridge later this week.) 

Added containsRefName to SequenceDictionary, allowing us to check whether a SequenceDictionary object contains a particular sequence.

SequenceDictionary recordNames keys should be Strings (they had been Avro Utf8 objects, which was messing up lookups into the map itself from Strings).