Update IGV to use new FeatureCodec API
Have MUTCodec load read header in constructor, as before
User LineIterator.peek() when reading headers so we don't read beyond header region
Override VCFWrapperCodec.decodeLoc to call through to wrapped Codec

The default implementation didn't work with VCFVariant, because we adjust the start positions of indels to remove redundant reference bases

IGV-1903
Merge branch 'TribbleVCF'